The pistachio nut ( Pistachia vera .) is one of the favourite tree nuts of the world and is widely cultivated in saline, dry and hot areas like Iran . proper harvest and good processing are the major basis of having good quality. Drying of agricultural products has always been of great importance for the preservation of food by human beings. Sun drying and cabinet drying are well-known food preservation techniques that reduce the moisture contents of agricultural product, and thereby prevents deterioration within a period of time regarded as the safe storage period. Since all fresh pistachios can not be consumed at the time of harvest, preservation provides a larger market, allowing consumers to buy the preserved pistachio around the year. Pistachio nuts may be dried using either solar energy or mechanical systems. Traditional sun-drying is a slow process compared with other drying methods, but it is said to give dried pistachios their distinctive quality. Simulation models are helpful in designing new or in improving existing drying systems or for the control of the drying operation. A forced convection solar dryer and a cabinet dryer were used in this research. Pistachio nuts were dried in the cabinet dryer with different temerature and air velocities (45, 50, 55 and 60°C, and 0.5, 1 and 1.5 m/s), solar dryer and open sun drying. Effective water diffusivity values ranged from 9.7 × 10 -10 to 4.58 × 10 -10 m 2 /s. The results showed that drying time in the solar dryer is shorter than open sun drying. Shrinkage and rehydration ability of food materials during air drying adversely affects the quality of the dried products. The experimental shrinkage data show a linear behavior with moisture content, with a reduction in sample volume with decreasing moisture content. The effect of temperature and air velocity on the shrinkage phenomenon can be neglected. The experimental data didn't show the dominating effect of temperature and air velocity on the rehydration ability of this product. The influence of temperature and air velocity on the rehydration ability can thus be neglected. The drying data as the moisture ratio (MR) versus drying time were fitted to the 13 thin layer drying models. The comparison criteria used to evaluate goodness of fit, namely the coefficient of determination (R 2 ), the root mean square error (RMSE) and the reduced chi-square (? 2 ). The Page model for cabinet dryer, the Page model for sun dryer and the two term exponential model for open sun drying gave higher R 2 and lower RMSE and ? 2 values than other models. Finally, a numerical model was developed to simulate the moisture transfer during thin layer drying of pistachio nuts by finite difference method on MATLAB software. The predicted wa
