High-molecular-weight atactic polypropylene (HMW-aPP) was used as the impact modifier in isotactic polypropylene (iPP). HMW-aPP (Mw 315,000 g/mol) was made by using metallocene catalysts, and iPP/HMW-aPP blends were prepared in a twin-screw extruder. The effects of HMW-aPP on the crystalline morphology, and thermal and mechanical properties of iPP were investigated by polarizing microscopy, differential scanning calorimetry, a universal testing machine, and Izod impact tests. The crystallization temperature and crystallinity of the iPP/HMWaPP blends increased compared to that of pure iPP. The HMW-aPP acted as a diluent during the crystallization of the iPP/HMW-aPP blends. These diluent HMW-aPP molecules prevented entanglement of the iPP molecules, which facilitated the crystallization of iPP. The Izod impact strength of the iPP/HMW-aPP blends increased with increasing HMW-aPP content owing to the incorporation of the soft HMW-aPP into the hard iPP. The Izod impact strength of the iPP-80 blend dramatically increased by 680% compared to that of neat iPP.