Abstract: This experiment carried out on three potato cultivar [Agria (susceptible), Satina (semi-tolerant) and Ceaser (tolerant to water deficit)] and four irrigation regimes (after 30 mm evaporation from basin class A; after 30 mm evaporation from basin class A with spraying by potassium humate; after 60 mm evaporation with spraying by potassium humate and after 60 mm evaporation from basin class A) for three locations of Ardabil (Alarog, Hassanbarog and Khoshkeroud) in Northwestern Iran, for two years (2007-2008). Experimental design was split plot with three replications. Potassium humate sprayed (250 mL ha-1) in the three stage of emergence, before tuberization and during tuberization period. The highest total and marketable tuber yield, plant height, tuber number and weight per plant had the lowest amount under stress conditions and the highest under normal and normal with spraying by potassium humate conditions. Use of potassium humate in water deficit condition increased tuber yield more than 0.93 and 9.63 t ha-1 under normal and stress conditions, respectively. Caesar cultivar had the highest total and marketable tuber yield, plant height, tuber weight per plant and dry matter percent under normal with spraying by potassium humate, stress with spraying by potassium humate and stress conditions and Agria cultivar under normal condition. Khoshkeroud and Alarog locations were produced the highest total and marketable tuber yield, plant height, tuber number and weight per plant.