In order to determine the critical period of weed control in safflower (Carthamus tinctorius L.), a field study was conducted in the Agriculture Research Station of Yasouj, Iran, during 2008-2009. The experiment carried out in a Randomized Complete Block Design (RBCD) with 16 treatments and four replications. Treatments consisted of two series of treatments. The first group consisted of weed removal from planting to 5-leaf, stem elongation, mid period of stem elongation, lateral stem emergence, beginning of flowering, end of flowering, end of seed ripening stages and then weed retaining until harvest. The second group consisted of weed retaining from planting to mentioned stages and then weed removal until harvest. Two control treatments (weed free and weedy check) were also applied. In order to determine the critical period of weed control, curve fitting Gompertz equation (for weed free period) and Logistic equation (for weed infested period) were used. Results showed that seed yield (31.4%), number of heads per plant (61.5 %) and number of seeds per head (23.9 %) were significantly decreased by increasing the weed interference period, and by increasing weed-free period, these characteristics were increased significantly. With considering 10% acceptable yield losses, the critical period of weed control of safflower was 134-188 days after planting (between early stem elongation and lateral stem emergence).