Melt grafting of maleic anhydride (MA) and alpha-methylstyrene (AMS) onto polypropylene (PP) was performed by reactive extrusion. Effects of AMS on the graft degree of MA, crystallization behavior, and thermal properties of the graft copolymer were investigated. Results show that the addition of AMS as a comonomer can efficiently improve the MA graft degree. When the molar ratio of AMS to MA is 0.9:1, the maximum MA graft degree is attained, which increases about 56% compared with that using single monomer of MA. The results of the graft degree of MA obtained by chemical titration (CT) agree well with those obtained by Fourier transform infrared spectroscopy (FTIR). Melt flow rate (MFR) measurements indicate that the addition of AMS effectively reduces the degradation of PP molecules. The wide-angle X-ray diffraction (WAXD) results show that in comparison with the PP-g-MA sample, the PP-g-(MA-AMS) sample shows no new crystalline form, but has a slight decrease in the average crystalline domain size. According to the results of thermogravimetry (TG) and differential scanning calorimetry (DSC), the graft PP in the presence of AMS exhibits a lower melting point and a higher crystallization temperature and thermal stability in comparison with that without AMS. POLYM. ENG. SCI., 52:814-819, 2011. (C) 2011 Society of Plastics Engineers