The cardanol grafted onto polypropylene (CAPP) was prepared by reactive extrusion with polypropylene (PP) and natural renewable cardanol, which could improve the inherent defects of PP such as chemical inertness and hydrophobicity. Moreover, the cardanol grafted onto PP could resolve the degradation of PP during the process of reactive extrusion, storage and application. In this paper, the yield strength, tensile strength, elongation at break of modified and unmodified PPs were tested during the aging process. A possible aging mechanim of PP and anti-aging mechanism of cardanol grafted onto PP were proposed. Results showed that the initiator dicumyl peroxide caused the degradation of PP chains during the aging processing. After being aged for 24 h, the yield strength CAPP0 decreased from 28 MPa to 15 MPa and the elongation at break reduced by 795%. CAPPs possessed outstanding anti-aging performance owing to the cross-linking and entanglement of the side chains of cardanol grafted onto PP. The yield strength, tensile strength and the elongation at break of CAPP5 changed lightly even when it was in long-term irradiation for 480 h. The CAPPs had better anti-aging properties than the mixture of PP and 5% of cardanol. CAPP containing 3% of cardanol could effectively prevent the degradation of the main chain polypropylene.