Grafting of maleic anhydride (MAH) onto polypropylene (PP) performed in microcolloid systems by coirradiation was studied in this paper. First, the microcolloid of PP and MAH in xylene was prepared and its dynamic light scattering (DLS) shows particle size distributions of microcolloid system in 4-6 mu m. Second, the effect of irradiation dose and monomer concentration on the amount of grafted MAH was investigated. The results show that the coirradiation in microcolloid systems can obviously cause an increase of the amount of grafted MAH. The percentage of grafting of the product amounts to 5.24%. The molecular structures of grafted MAH prepared were characterized by Fourier-transform infrared spectroscopy. Differential scanning calorimetry and wide-angle X-ray diffraction were used to determine the degree of crystallinity and crystalline structure.