Abstract: A series of nano-ZnO/polypropylene (PP) composites were prepared via melt blending. The effect of compatibilizer maleic anhydride grafted polypropylene (PP-g-MAH) on nucleation and crystallization behavior, crystal structure, morphology and mechanical properties of nano-ZnO/PP composites was investigated. The results show that nano-ZnO has good effect on the β-crystal nucleation of PP when nano-ZnO is added in low level (mass fraction less than 5%). When the mass fraction of nano-ZnO is more than 5%, nano-ZnO has obvious heterogeneous nucleation effect on PP matrix, with the cryatallization peak temperature of PP shifted to higher temperature significantly, and PP presents preferential growth in (040) crystal face. PP-g-MAH enhances the interfacial interaction between nano-ZnO particles and PP matrix, which improves the dispersion of nanoparticles, and promotes the heterogeneous nucleation of PP matrix. Consequently, the tensile strength and impact strength of nano-ZnO/PP composites are improved obviously. However, the ability of β-crystal induced by nano-ZnO is inhibited. The toughness of nano-ZnO/PP nanocomposites improved due to better interaction interface is significantly stronger than the β-crystal toughening effect induced by nano-ZnO.