Abstract: With the aid of fundamental correlations of statistical mechanics and some assumptions,a simple method for calculating pseudocritical constants of mixtures containing nonpolar and slightly polar components is proposed in this paper. A detailed comparison of pseudocritical constants by this method with that by Lee-Keslers rule has been made. Finally, using this method together with Lee-Kesler tables, compressibility factors for some gas mixtures have been calculated, obtaining good agreement with experimental data.