Drought stress is one of the most important abiotic stresses influencing performance of crop plants. Therefore, the identification or development of tolerant genotypes is of high importance for incorporating in cotton production. In this study to evaluate the effect of drought stress on some cotton traits, 5 improved cotton varieties were studied in a split plot design with three replications in 2 years (2000-2001) at 2 locations (Hashemabad and Anbaroloom); one with Mediterranean climate and the other with drought-stress condition. Treatments were irrigation as main plot in 3 levels (I0=without irrigation, I1=one time irrigation; that carried out 70 days after sowing, and I2=at least 3 times irrigation) and varieties as subplot in 5 levels (5 genotypes). In the basis of combined variance analysis significant differences were detected among varieties for yield, boll number, boll weight, length and number of sympodial and monopodial branches. Drought stress decreased yield, boll number, boll weight, and induced earliness. With increasing irrigation frequency, earliness lightly reduced in the former climate probably because of inducing vegetative growth and retarding in generative phase. In latter climate increased irrigation frequency had a positive effect on the yield. It seems that water deficiency has reduced yield via decreasing boll number. The number of formed bolls in stressful conditions was less than that of in non-stressful conditions. Stress tolerance index (STI) revealed that Siokra-324 and Tabladila were more tolerant and stable varieties.