Globally fish consumption is increasing due to increase in awareness of its health benefits. Fish is a rich source of protein, carbohydrates, minerals, vitamins and fats. Among other fatty acids (including Long chain polyunsaturated fatty acids) omega-3 fatty acids, especially eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are beneficial for the human health. This thesis investigated the differences in the chemical composition of five economically important and most consumed fish species in Pakistan, namely Mahseer (Tor putitora), Rainbow trout (Oncorhynchus mykiss), Silver carp (Hypophthalmichthys molitrix), Thela fish (Catla catla) and Common carp (Cyprinus carpio). These species were evaluated for heavy metals accumulation, chemical composition, fatty acids composition, storage stability of their oils under room temperature and high shier conditions after gamma irradiation. Further, fatty acids composition of commercially available fish oil supplements and their oxidative stability was also evaluated to compare with their fatty acids profile to the aforementioned fish oils. There was a significant difference for most chemical components and fatty acids (P < 0.05) in the analyzed fish species. The highest polyunsaturated fatty acids (PUFAs) content was obtained from the Rainbow trout (24.63%), followed by Thela fish (22.52%), Common carp (19.52%), Mahseer (16.25%), whereas the lowest value was found in the Silver carp (15.20%). Similarly, a significant difference in metals content of all fish species was observed. Interestingly, heavy .metals concentration was much higher in fish liver and skin than that estimated in gills and muscles tissues. Furthermore, effects of irradiation on the storage stability of various crude fish oils extracted from selected fish species were tested for physicochemical constants, i.e., peroxide value (POV), free fatty acid value (FFA), anisidine value (AV), iodine value (IV), color and β-Carotene. Among the fish oils examined Common carp followed by Rainbow trout oils showed the greatest stability against oxidation. Additionally, fish oil capsules were evaluated for fatty acids profile to understand what is consumed to fulfill nutritional requirement regarding fish oil viii supplementation study about quality evaluation demonstrates that the tested samples of fish oil capsules show large variation of long-chain omega-3 polyunsaturated fatty acids and contain fewer amounts as claimed on the labels of the products contains considerably oxidized and has cost as well as health implications for the consumers. The results presented in this thesis will be beneficial for the nutritionists, physicians, consumers and researchers who are motivated to improve the nutritive value, processing and marketing of selected fish species and fish oil supplements.