Moisture sorption isotherms for pistachio powder were determined by gravimetric method according to COST 90 project at temperatures of 15, 25, 35, and 40oC . Maximum hysteresis level in sorption isotherms curve was observed at the range of water activity 0.2-0.7 . On the other hand hysteresis was diminished between adsorption-desorption isotherm curves when temperature reached 40oC. Some mathematical models were tested to measure the level of fitness of the experimental data including BET, GAB, Smith, Oswin, Halsey and Henderson.The mathematical analysis proved that Smith model was the most appropriate one.Thermodynamic relationship of Clasius-Clapeyron was applied to determine the isosteric heat of adsorption and desorption of pistachio powder. Finally, critical points of equilbrium moisture content for pistachio powder were found using adsorption-desorption isotherm curves and GAB equation in the range of 15-40oC.