This paper presents a mathematical modeling of thin layer forced and natural solar drying of shelled and unshelled pistachio samples. In order to estimate and select the suitable form of solar drying curves, 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 (r, chi(2)), which were predicted by non-linear regression analysis using the Statistica Computer Program. 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 by considering the coefficients of determination, r(sfsd) = 0.9983, chi(sfsd)(2) = 2.697 x 10(-5); r(ufsd) = 0.9990, chi(ufsd)(2) = 1.639 x 10(-5) for thin layer forced solar drying and r(snsd) = 0.9990, chi(snsd)(2) = 3.212 x 10(-6); r(unsd) = 0.9970, chi(unsd)(2) = 4.590 x 10(-5) for thin layer natural solar drying.