Bulky materials reclaimed from waste plastics are not adequate to use as structual materials since mechanical properties of them are relatively low. In the present work polypropylene (PP) films, which are a potential main component of waste plastics in near future, were irradiated with gamma rays in acetylene gas atmosphere and then were injection molded to PP test specimens to measure its mechanical properties. The flexural strength and modulus of elasticity in bending have been found to increase with irradiation dose. Those values reach the maximum at the irradiation dose of 40-80kGy. In the case of the gas pressure of 199 kPa and the irradiation of 57 kGy, the observed strength and modulus of elasticity of the PP specimen are 59MPa and 2, 050MPa, respectively, which are 1.5 times and 1.7 times as large as those of unirradiated PP. The tensile properties of the PP Specimen also are improved by 1.4 times of unirradiated PP. The acetylene gas pressure has little influence to improve the properties in a range above 100kPa. The injection molding after the irradiation is considered to be effective to improve the mechanical properties of the PP specimen since the heating and mixing of irradiated films enhance the crosslinks of acetylence to PP structures and the network formation.