Abstarct: The critical period of weed control is the portion of the life cycle of a cropduring which it must be kept weed-free to prevent yield loss due to weedinterference.In order to determine critical period of weeds control in early maturing cotton Golestan cultivar in delayed planting date a field experiment was carried out in CottonResearch Station of Hashem-Abad, Gorgan in 2014. The experiment was carried out as a randomized complete block design with three replications and 14 treatments.The trial included fourteen treatments consisted of seven initial weed-free periods in which plots were kept free of weeds for 0, 100, 200, 300, 450,600 and 750 growth degree days (GDD), and then weeds were allowed to grow until harvest and control (Weed free) treatment. The second group of treatments were included seven initial weed-infested periods in which, weeds were allowed to grow for all of seasin (weed infest), 100, 200, 300, 450, 600 and 750 growth degree days (GDD), after which the plots were kept free of weeds until harvest. Weed sampling carried out in first series of treatments at the end of growth season and in second series of treatments at theend of infested periods. Survey results showed that dominate weed species were Cyperus rotundus, Xanthium strumarium, Physalis alkekengi, Euphorbia helioscopia, Chrozophora tinctoria and Portulaca oleracea. Among recognized weeds species, Cyperus rotundus with 35.87 and Solanum nigrum with 1.35% had the maximum and minimum dry matterrespectively. Results showed that, total density and dry weight of weeds increased as the duration of weed infested period increased and it was decreased with increasing duration of the weed-free period.The yield of cotton increased with increasing duration of the weed-free period and decreased as the duration of weed infested period increased.Cotton yield increased 54.5 % in weed free treatment than weed infested treatment. The critical period for weed control in cotton baxsed on a 5, 10 and 15% acceptable yield loss level was calculated by fitting logistic and Gompertz equations to cotton yield data.The results showed that beginning of CPWC ranged from 110, 200 and 240 GDD, at 5, 10 and 15% acceptable yield loss (AYL), which equates to 8, 16 and 19 days after cropemergence, respectively. The end of the CPWC varied from 760, 660 to 570 GDD, at 5, 10 and 15% AYL, which equates to 72, 57 and 50 days after crop emergence, respectively. Results of this study suggest that cotton should be kept weed free from 200 to 660 GDD orphonological stages of 3-5 true leaf to Onset flowering (16 to57 days after crop emergence), to avoid yield losses in excess of 10%.