This paper presents a method for calculating the effective plastic properties of granular composites based on the model of deformation of porous materials. A distinctive feature of the method consists in averaging the microscopic stress and strain is not like the full volume, but as the effective volume of the phases. Effective volumes of averaging are found by solving the boundary problems of plastic deformation of simplified structural models of two-phase composites. The comparison between calculation results and experimental data at the construction of stress-strain diagrams for three types of granular composites: with polymer and metal matrix, with pseudo-alloys and with ceramic-metal alloys, was carried out.