A field study was carried out to determine the critical period of crop weed competition in cotton (Gossypium hirsutum. L.) during the summer season (Febuary to July) of 2007. Twelve treatments consisting of weed free and weedy periods at 20, 40, 60, 80 and 100 DAS and at harvest along with a weedy and weed free check were arranged in randomized block design with three replications. The predominant weed species in the experimental field were Echinochola colona, Leptochloa chinensis, Cyperus rotundus, Trianthema portulacastrum, Rotala densiflora, Eclipta alba and Phyllanthus maderaspatensis. Seed cotton yield loss increased in the duration of competition and maximum loss (96.5%) occurred due to full season competition. Seed cotton yield increased significantly with the increase in initial duration of weed free condition upto 80 DAS. Beyond 80 DAS, seed cotton yield was reduced considerably due to lower boll setting percentage. Critical period of weed competition was found to be 20 to 60 days after sowing