Polypropylene (PP) is a kind of semi-crystalline polymer so it is hard to foam with supercritical carbon dioxide (SCCO2). We used polypropylene block (PP-B) copolymer as a modifier to improve the crystallization behaviors and foaming performance of polypropylene homo polymer (HPP). HPP, PP-B, and a series of HPP/PP-B blends were characterized by differential scanning calorimeter (DSC), X-ray diffraction, and polarized optical microscope. Results show that both the crystallization behaviors and melt strength have influence on the cell structure. The crystallization temperature of PP-B is about 7 degrees C higher than that of HPP and that the crystallization behavior of HPP/PP-B blends is similar to that of PP-B. Much denser and smaller size spherulites are observed in PP-B and HPP/PP-B blends than in HPP, and the crystal structure is unchanged after blending. Scanning electron microscope results show that much more uniform, smaller cells can be obtained for the HPP/PP-B blends. The crystal nuclei formed earlier can act as physical crosslink points, increasing the melt strength and improving dramatically the cell structure and morphology of the HPP/PP-B blends. Furthermore, the best cell structure and morphology was achieved for HPP/PP-B blends with the ratio of 70/30 under the same foaming conditions. POLYM. ENG. SCI., 56:1175-1181, 2016. (c) 2016 Society of Plastics Engineers