The melting behavior of blends of polypropylene (PP) and maleic anhydride grafted PP (mPP), crystallized both non-isothermally and isothermally, have been investigated by differential scanning calorimetry (DSC). Also the relationship between co-crystallization, phase separation and the corresponding crystallization conditions in the blends is presented. It has been found that either co-crystallization or phase separation in the blends can be obtained, depending on the crystallization conditions. If the cooling rate is very slow in the non-isothermal case, or the crystallization temperature is relatively high in the isothermal case, phase separation between PP and mPP molecules will be observed in the blends. Otherwise, co-crystallization between different molecules will dominate in the blend system. An effort was made to elucidate these phase phenomena with the aid of kinetic parameters by isothermal crystallization. Fractional values of the Avrami exponent are obtained, which range from 1.93 to 3.21, indicating an average contribution of simultaneous occurrence of various types of nucleation and growth of crystallization. The half-life t(1/2) Of crystallization shows that incorporation of mPP in PP can either have little effect or greatly increase the crystallization rate of PP, depending on the crystallization temperatures. At higher crystallization temperatures, the crystallization rate of the blends is higher than both the pure homopolymers, resulting in phase separation morphology. The results by small angle X-ray scattering (SAXS) are consistent with those of the DSC measurements. It has been concluded that the crystallization kinetics, not the thermodynamics, mainly determine the final phase morphologies in this particular system.