A process for producing a modified propylene polymer comprising the steps of premixing a propylene polymer with a vinyl monomer and polymerizing the vinyl monomer by kneading the premix at a temperature of a melting point thereof or higher is described, wherein the propylene polymer is a particulate propylene polymer having a specific surface area of 0.1 m.sup.2 /g or more. A modified propylene polymer composition is also described, obtained by mixing (a) a particulate propylene polymer having a specific surface area of 0.1 m.sup.2 /g or more, (b) 5% by weight or more of a vinyl monomer mainly comprising an aromatic vinyl monomer based on the total weight of the vinyl monomer and the propylene polymer, and (c) a radical polymerization initiator, substantially impregnating the vinyl monomer and the radical polymerization initiator into the propylene polymer, and then polymerizing the vinyl monomer by melt-kneading.