The effect of propylene-ethylene copolymers (PEc) with different ethylene-unit contents on melting and crystallization behaviors of isotacticpolypropylene (iPP) were investigated by differential scanning calorimetry (DSC) and polarized light microscopy (PLM). The results show that the addition of PEc decreases significantly crystallization temperature (T-c) of iPP, but slightly affects melting temperature (T-m). With increasing the ethylene-unit content of the propylene-ethylene copolymers, the decrease in crystallization temperature of iPP is smaller. The PLM results show that the spherulite growth rate decreases with increasing crystallization temperature for iPP and iPP/PEc blends. The higher the ethylene-unit content of the copolymers is, the lower the spherulite growth rate (G) of iPP/PEc blends is. The influence of the PEc on nucleation rate constant (K-g) and fold surface energy (sigma(e)) of iPP was examined by nucleation theory of Hoffman and Lauritzen. The results show that both K-g and sigma(e) of iPP/PE20(80/20) and iPP/PE23(80/20) blends are higher than those of iPP, demonstrating that the overall crystallization rate of iPP/PEc blends decreased as compared to that of iPP, resulting from the decrease of the nucleation rate and the spherulite growth rate of iPP. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 116: 1172-1183,2010