Structural changes in polyethylene terephthalate (PETP) modified with segmented polyurethanes (SPU) contained various ratio of flexible and hard segments, are studied. The data on microcalorimetry, N diffusion, CHCl3 sorption from n-pentane, and isothermal crystallization kinetics, are considered. SPU modifiers accelerate the PETP crystallization from melts or gassy state, and promote more perfect structure formation.