A field experiment was conducted for two consecutive cropping seasons 2012 and 2013 at Shandaweel Agricultural Research Station, Agricultural Research Center, (ARC), to estimate the critical period of weed competition and yield loss in sesame. Quantitative series of both increasing duration of weedy and weed free periods were compared with complete weed free and weedy check. The experiment included 14 treatments of weed competition which seven of them were weed-free period at 2, 4, 6, 8, 10, 12 weeks after emergence (WAE) and whole  season weed free, and seven treatments of weed competition at 2, 4, 6, 8, 10, 12 (WAE) and whole  season weed competition. The experimental design used was randomized complete block design (RCBD) with four replications under naturally weed infestation. The major weeds were Xanthium strumarium L., Portulaca oleraceae L., Euphorbia geniculata L., Corchorus olitorius L. and Tribulus terrestris L. as broad- leaved weeds and Echinochola colonum L. and Digitaria sanguinalis L. as grassy weeds.