Equal channel angular pressing (ECAP) invented by Segal was applied successfully to metal process of aluminum and magnesium alloys for increasing their strength by fining the grain size. But a little attention has been paid to its application to polymers. In this paper, ECAP process of crystalline polymer, polypropylene (PP), was studied using optical microscope, differential scanning calorimetry (DSC), specific gravity measuring system and small tensile testing machine. It was shown that 1. ECAP was effective at a temperature slightly lower than the crystallization temperature and at a low pressing speed, 2. the shear deformation was very intense at the interior of the specimen, while the surfaces was not sheared and 3. the yield stress of PP along the extrusion direction increased with anincrease in the repeated numbers of ECAP process.