Pakistan has made tremendous progress in majority of the food crops however country is suffering from deficit of quality edible oil due to unavailability of high yielding cultivars and deficit of irrigation water. This situation demands the development of high yielding and drought tolerant oilseed cultivars. During a breeding program for improved cultivars, the knowledge of combining ability and gene action is important. Therefore, this study was undertaken to examine combining ability and inheritance pattern of essential characters in Brassica napus under irrigated as well as rainfed conditions at the University of Agriculture Peshawar, Pakistan. During crop season 2010-11, eleven Brassica napus advance lines were crossed with four genotypes following line × tester matting design. The resultant 44 F1 crosses were planted in the field along with their parental genotypes for evaluation during crop season 2011-12. Data obtained regarding morpho-yield and oil quality traits were graphically analyzed for combining ability among genotypes following GGE-biplot methodology to identify best combiners. On the basis of performance, two testers and two lines along with their four crosses were selected and forwarded to develop their F2s, BC11 and BC12 during 2012-13. The resultant generations were evaluated under irrigated and water deficit conditions under rainout shelter as well as field condition during 2013-14. Inheritance pattern of various important traits via generation mean analysis was studied. The results obtained from parental and F1 crosses data indicated that GCA effects were comparatively higher than SCA effects for days to 50% flowering, primary braches plant-1, number of pods on main raceme, 1000 seed weight, seed yield plant-1, erucic acid and glucosinolate content, indicated the importance of additive type of gene action for the expression of these traits in the present set of genotypes. Desirable negative GCA was depicted by L-6, T-2 and T-3 for days to flowering. Both GCA and SCA were found important with predominant role of SCA for plant height where parental lines; L-3, L-4, L-6, L-7 and L-8 and tester T-4 showed positive GCA effects while cross combinations (L-8 × T-2), L-3 × (T-4 and T-1) and (L-7 and L-6) × T-3 were identified outstanding. For primary branches per plant L-6, L-7, T-1 and T-3 were found best general combiners. Regarding number of pods on main raceme line L-4 and L-7 produced good combinations 22 with testers T-1 and T-2 whereas, L-6 and L-8 resulted in superior hybrids with tester T- 3 and T-4. For 1000-seed weight, L-6, L-7, L8, T-1 and T-3 showed maximum positive general combining ability. For seed yield plant-1 lines L-6 and L-7 were identified as best specific combiners with T-3 and T-4 respectively. Additive genetic control mechanism was found more important in controlling oil content in the present set of genotypes. Among parents, L-6, L-7, L-4, T-1 and T-4 were best general combiners for oil content. For erucic acid and glucosinolate content, parental genotypes (L-6, L-7, L-5 and L-8) depicted desirable negative GCA. These lines also produced the most desirable cross combinations especially with tester T-1 and T-2. Based on the results obtained from combining ability studies of important traits, two lines, L-6 and L-7 and two testers, T1 and T-3 were identified as the most promising parental genotypes. Therefore, these four parents and their resultant four F1 crosses were used in the following season (201213) to develop four F2, four BC11 and four BC12 generations. The resultant generations were evaluated under irrigated and rainfed conditions for inheritance studies via generation mean analysis approach at seedling and whole plant stage during crop season 2013-14. Inheritance studies at seedling stage explored both additive and non-additive type of gene action along with non-allelic interaction for relative water content. Minimum reduction in relative water content due to drought stress was observed in parental genotype L-7 and T-3. Additive type of gene action under irrigated as well as rainfed conditions was observed for proline content. Maximum increase in proline content in response to drought stress was observed in parental genotypes L-7 and T-1 and their segregating generations. Overall, dominance type of gene action along with dominance × dominance epistasis was involved in controlling chlorophyll content. Least reduction in chlorophyll content was observed in parental genotype T-3 and in segregating generation of L-7 × T-3. The Genotype × trait biplot explored strong and positive relationship of proline and chlorophyll content was observed with seed yield and associated traits under irrigated as well as drought stress. 23 Inheritance study under field conditions for morpho-yield and oil quality traits revealed that additive type of gene actions along with epistasis were involved in the expression of seed yield plant-1, glucosinolates and erucic acid content. Dominance type of gene action along with epistasis was mostly involved in controlling days to flowering, Pod length and seed pod-1, 1000 seed weight and oil content. Dominance type of gene action was found for primary branches palnt-1 except two L-6 × T-1 and L7 × T-3 under rainfed condition. For pods on main raceme, in most of the crosses dominance type of gene action was observed except L-7 × T-1 under rainfed condition, which depicted additive type of gene action. Simple selection in early generation would be effective for traits controlled by additive types of genes whereas selection should be delayed till advance generation for traits controlled by dominance type of genes. Moreover a change in magnitude of gene action was observed for plant height with a change from normal to rainfed condition. Under such circumstance separate selection criteria should be followed for each environmental condition. Regarding high seed yield per plant and low erucic acid the F2 generation of L-7 × T-1 might be used for selection of potential segregants. For low erucic acid and glucosinolates having additive type of gene action, the segregating generations of cross combination L-6 × T-1 might have potential segregants for early generation selection. For incorporation of drought tolerance and high seed yield both proline and chlorophyll content can be used as a selection criterion.