This study clarifies and quantifies factors which increase the ductility of a low-molecular-weight propylene homopolymer having an intrinsic viscosity of 0.89 dl/g. The tensile behavior of homopolymer/ethylene-propylene rubber (EPR) blends was studied from the viewpoint of the associated molecular structure of EPR and its compatibility with the homopolymer. When EPR is "dissolved" in a homopolymer, the glass transition temperature (T-g) of the amorphous phase of a homopolymer was found to shift to a lower temperature, with homopolymer/EPR compatibility being subsequently evaluated using this shift, i.e., Delta T-g. Results show two conditions are required to improve the ductility of the low-molecular weight propylene homopolymer : Delta T-g must be greater than or equal to 3 degrees C and greater than or equal to 30 wt% EPR must be blended with the homopolymer.