Critical period of crop/weed competition was studied in upland rice during monsoon season from 1981 to 1983.Echinochloa colonum, Seirpus grossus, Dactyloctenlum aegyptium, Cyperus rotundus, C. iriaandTrianthema monogynawere the major weed species. Competition from weeds during the first 15 days after sowing (d.a.s.) had no significant effect on the grain yield of rice. Competition beyond 15 d.a.s. caused drastic reduction in the grain yield. Grain yield of rice Increased significantly with the increase in the duration of weed‐free period up to 45 d.a.s. during two years. However, further increase in the weed‐free period up to 60 d.a.s. caused significant improvement in the grain yield of rice in only one year. Weeds emerging after 45 d.a.s. were lower in density and their growth was suppressed by the crop. Density of weeds emerging between 15 and 30 d.a.s. was high and could compete with the crop resulting in reduced grain yields. The period during 15 to 45 d.a.s. was found to be the most important for crop/weed competition.