The compositional heterogeneity, morphology and melting behavior of two commercial high-impact polypropylene (HiPP) in-reactor alloys were systematically investigated by the combined use of thermal fractionation, Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). Correlations between the mechanical properties and the molecular architectures and phase morphologies were discussed. The results show that the mechanical properties of the alloys are strongly influenced by the contents of ethylene-propylene rubber (EPR) and ethylene-propylene block copolymer (EPC), intrinsic viscosity ratios of EPR and the matrix, and the morphology in polypropylene in-reactor alloys. The EPC component acts as a bridge connecting the dispersed amorphous phase and crystalline polypropylene matrix. The advanced impact toughness of the HiPP samples can be interpreted as a consequence of their high EPR content and the good compatibility between the different components in the multiphase system. When the xylene soluble (XS) component was 20 wt% and an intrinsic viscosity ratios (λ) of xylene soluble component and xylene insoluble (XIS) component was1.6, the impact strength and flexural modulus of the HiPP samples were higher than 50 kJ/m 2 and 900 MPa, respectively.