A novel method for the tailoring of unique three-phase crystalline systems in isotactic poly(propylene) has been proven. It is based on a synergistic application of a specific beta-nucleating agent and high pressure during crystallization. The formation of a gamma phase was supported by elevated pressure and high temperature during crystallization; under these conditions the growth of both beta and alpha phases Was significantly suppressed. Nevertheless, during the course of crystallization at lower pressure and/or lower temperatures, strong beta-nucleation efficiency favored the formation of a beta phase.