In order to determine the critical period of weed control (CPWC) in Corn (Zea mays L.), an experiment was carried out in a randomized complete blocks design with 14 treatments and three replications during 2013 growing season in the Nahavand climatic conditions. Two series of the treatments including weed-free and weed-infested were applied in the regular time distances of 10, 20, 30, 40, 50 and 60 days after Corn emergence. Two check treatments including full and no control of weed during the whole season were also included. The nonlinear models kind of Logistic and Gompertz were applied to determine the start and end of critical period of weed control respectively. Major weed species were field bindweed (Convolvulus arvensis L.), red root pigweed(Amaranthus retroflexus L.), pigweed(Amaranthus blitoides L.) and lambsquarters(Chenopodium album L.) with greater biomass and size. The results showed that periods of weed- free and weed- infested differently affected the dry weight and number of weeds in all treatments. The critical periods of weed control in Corn by accepting a 5% acceptable yield loss, it is necessary to control weeds in a period between 14-47 days after planting or 117- 566 growth degree day(4-12 leaf stage) and by accepting a 10% acceptable yield loss it is necessary to control weeds in a period between 23-36 days after planting or 220- 419 growth degree day(6-10 leaf stage).