This experiment was carried out during the seasons 2002/2003 , 2004/2005 at El-kod experimental farm of El- kod Agricultural Research Station. The study aimed to determine the critical period for application of supplemental irrigation and its effect on yield of sesame crop, Randomized Complete Block Design (RCBD) was used with seven replications, the experiment included 3 treatments: (A) supplemental irrigation applied at the beginning of flowering stage (B) supplemental irrigation applied at the end of flowering stage and beginning of yield formation stage (C) without any supplemental irrigation (farmer's treatment). Results indicated that highest yield was obtained in treatment (A) in which supplemental irrigation applied at the beginning of flowering stage and differed significantly at 5%level as compared to the farmer's treatment (C) in both seasons of the experiment, followed by treatment (B) in which supplemental irrigation was applied at the end of flowering stage and beginning of yield formation stage. Highst water use efficiency (0.132-0.154 kg/m3) was obtained also in treatment (A) during the both seasons of study.

Key Words :Irrigation, Supplemental Irrigation, Sesame, Critical Period