The micro-morphology of PP/HDPE/EPDM ternary blends is studied with the help of SEM in this paper. The results show that EPDM and HDPE are in the form of core-shellstructure in PP matrix. This structure results in the properties of excellent toughness and strength. Based on the observation of the crazes and shear bands,the mechanism of HDPE and EPDM' s synergistic toughening PP is explained.