The demand for using polypropylene in automobile interior trim parts has remained high in Japan because of its light weight and low cost. To give polypropylene the high heat resistance necessary for use in parts requiring it, we have improved on the product itself and have developed optimum fastening and joining structures that permit a wider application range. Specifically, in applying polypropylene to vehicles, copolyacrization using ethylene is adapted to eliminate the difficulty in achieving iapact resistance at low temperatures when using conventional high-crystallinity polypropylene (HCPP) alone. The filler employed was carefully selected to improve heat and scratch resistance as well as stiffness. Furthermore, the fastening methods, firmness or flexibility of fit, and actual joint structures that are greatly affected by heat deflection were critically examined and improved to ensure maximum HCPP adoption. These developments have led to ABS replacement, thus greatly helping to reduce cost. This paper reviews these improvements, and introduces specific examples of HCPP applications to automobile interior trim parts.