A field study was carried out in the experimental farm of the Pandit Jawaharlal Nehru college of Agriculture and Research Institute, Karikal to determine the critical period of crop weed competition in zero - till cotton (Gossypium hirsutum L.) during the summer season (Febuary to July) of 2007. The higher dry weight (3514 kg ha-1) of weeds were observed in weedy upto harvest, throughout the crop growth period. The dry weight of weeds decreased substantially with increase in the duration of weed free period. Keeping the field weed free for first 40 days reduced the weed dry weight by 50 per cent. The yield components like number of bolls plant -1 and boll weight could be enhanced by 7.2 to 7.4, 14.3 to 18.1 and 4.8 to 5.2 times, respectively by maintaining the field weed free for first 60 DAS or more. However, weeding beyond 80 DAS significantly reduced the boll setting percentage. Seed cotton yield loss increased in the duration of competition and maximum loss (96.5 per cent) 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 considerably reduced the seed cotton yield due to lower boll setting percentage. Critical period of weed competition was found to be 20 to 60 days after sowing.