eight different mathematical models, which are semi-theoretical and/or empirical,were applied to the experimental data and compared according to their coefficients of determination,which were predicted by non-linear regression analysis using the Statistica computer program,for estimating and selecting the suitable form for solar drying curves and it was deduced that the logarithmic model could sufficiently describe thin layer forced solar drying of shelled and unshelled pistachio ,while the two-term model could define thin layer natural solar drying of these products in evaluation