A field experiment was conducted during summer season of 2006 and 2007 under irrigated condition in a sandy loam soil at Sriniketan, West Bengal to determine the critical period of crop-weed competition in sesame. Results revealed that weed-free condition between 15 and 45 days after sowing (DAS) significantly increased the yield of sesame, whereas the lower yield of was recorded when weeds were allowed to grow during this period. The maximum competition between crop and weed was between 15 and 45 DAS, which can be considered as critical period of crop-weed competition. However, the extrapolated critical period was determined from 19-42 DAS. To avoid the yield loss, weed management should be done in such a time so that minimum weed infestation is achieved in summer sesame.