Abstract: This experiment was done on the three potato cultivars [Agria (susceptible), Satina (semi-tolerant) and Ceaser (tolerant to water deficit)] and three irrigation treatments (after 30 mm evaporation from basin class A, after 60 mm evaporation+spraying by Potassium Humate, and after 60 mm evaporation from basin class A) for two locations in 2007. Experimental design was Split Plot with three replications. Potassium Humate spraying (250 mL ha-1) were done in three stages of emergence, before tuberization and during tuberization period. Combined analysis of variance showed that there were significant differences between locations, cultivars, irrigation treatments and locationxcultivars interaction as effect on tuber yield. Comparison of means for irrigation treatments showed that spraying by Potassium Humate in stress condition induced increasing of tuber yield. Spraying by Potassium Humate in water deficit condition increased tuber yield up to 11.01 ton ha-1. Ceaser had the highest tuber yield. It had higher tolerance to water deficit as well. Ceaser had a high potential in control and severe stress. Decrease in yield of Ceaser after 60 mm evaporation+spraying by Potassium Humate and after 60 mm evaporation from basin class A, relative to control (after 30 mm evaporation from basin class A) was 1.03 and 13.08 ton ha-1 but for Satina was 7.83 and 16.61 ton ha-1, respectively. Satina had the lowest Environmental Variance, Environmental Variance Coefficient, Finlay and Wilkinson`s and Eberhart and Russell`s model and was the most stable cultivar. Lin and Binns parameter showed that Ceaser and Satina were the most stable cultivars. Results of GMP, STI and MSTI were very considerable and Ceaser and Satina had a high yield in water stress and control conditions.