Weeds are the serious problem in our agriculture system. Therefore we need to know weeds and its relation to crops. Present study was conducted to describe the allelopathic effect of the three common weeds of Sindh Chenopodium album, Chenopodium murale and Anagallis arvensis on six tests crops viz., wheat, cowpea, chick pea, mung, pearl millet and maize. The experiments were carried out in laboratory and green house. The influences of weed on test species are evaluated by two separate laboratory experiments. In first trial study the comparison of osmotic stress by mannitol solution and allelopathic effect of weed extract with varied concentration (1, 3, and 5%) compared with different osmotic potentials (22, 32.9, 43.9 and 66kPa) of mannitol solutions, to explore the role of osmotic potential. The second experiment was conducted to study the allelopathic effect by shoot and root leachate of weed species on different parameters of test species. In the green house experiments monoculture (only test species and only weed species) and combine culture (Weed and test species) pot trial carried out. Allelopathic effects of weed from first experiments to last on these growth parameters i.e. percent germination, speed of germination, radicle length, plant height and fresh/dry weight of root and shoot of test species, the results obtained are summarized as under. 1. Comparison between osmotic potential and weed extract trial show that germination was not considerably effected by osmotic potential while in extract (1, 3, and 5%) Chenopodium album and Chenopodium murale highly reduced than the Anagallis arvensis. Chenopodium album (5%) extract reduced the percent germination (18% in cowpea, 30% in pearl millet) like Chenopodium murale 5% extract showed the similar trend (reduction, 22% in wheat, 50% in gram). 2. Low osmotic potential have no considerable effect on radicle growth of all test species as compared to extract trial. Radicle length significantly effected in comparative trial show the same trend in high osmotic potential 66 kPa = 5% extract of weeds. Five percent extract of Chenopodium album and mannitol solution (66 kPa) were significantly reduced the radicle length as 8.21 to 0.79cm in wheat and 7.84 to 1.57cm in gram. Same concentration (5%) of Chenopodium murale, Anagallis arvensis and mannitol solution were also significantly reduced the radicle length as 8 to 4cm in wheat, 8 to 1 cm in gram and 9 to 1 cm in pearl millet while in other weed as 10 to 3 cm in cow pea, 9 to 3 cm in pearl millet. On the basis of these results it is concluded that weed extract had pronounced and dominating effects on test crops by allelopathy which osmotic pressure also play a minor role 3. Root and shoot leachate of the weeds were considerably affected germination and speed of germination of test species. Radicle growth in some test species significantly affected by root leachate and shoot leachate. However leachate of Chenopodium album and Anagallis arvensis have no significant effect on radicle elongation of some test species i.e. gram, maize and pearl millet. Generally these results revealed that the root leachate highly effected the growth of test species than the shoot leachate. 4. In mono and combine culture study the morphological characters i.e. plant height, fresh and dry weight statistically assessed. The weed species also considerably effected seed germination of all test species in combine culture trials. Gram seed germination highly effected to all test species in Chenopodium murale (38%), similarly Chenopodium album also effected gram seed germination (72%) and Anagallis arvensis effected (70%) in gram and wheat while the remaining test species were also considerably affected. Further more in combine culture trial Chenopodium album reduced the plant height of gram (17.49cm), wheat (12cm) and pearl millet (17cm) significantly. Chenopodium murale and Anagallis arvensis also significantly effected on wheat (18, 12 cm), gram (8, 17cm) and mung (8, 14cm) while the remaining test species also significantly reduced the plant height. Shoot and root weight of test plant were found to be higher in weed free (control) trial. Dry weight of shoot and root of test species in combine culture growth were significantly reduced. Results showed that reduction in germination, plant height and dry matter were affected due to the allelopathic effect of weed species. Above investigations show that in general these weeds have allelopathic effects on test crops, in there initial stage of growth (germination and radicle elongation). Therefore precautionary action should be taken to remove these weeds from the field to increase crop production.