This paper presents the method of plasticity index calculation of isotropic two-process surfaces on the basis of index developed by Greenwood and Williamson for random surfaces of Gaussian ordinate distribution. This method was applied to computer generated isotropic two-process textures. The proposed plasticity index of two-process surface was compared with that of plateau surface and of the whole surface calculated by traditional manner. It was shown that plasticity index and generally the contact properties of two-process surfaces depended not only on plateau but also in a smaller degree on valley roughness. The method of estimation of summit radius of plateau surface from the analysis of two-process surface was presented too.