Free radical grafting with methyl methacrylate onto molten polypropylene was investigated in both an internal mixer and a modular co-rotating twin-screw extruder. There has been little open literature on melt free radical grafting copolymerization of methyl methacrylate. There is also little information on the evolution of grafting reaction with respect to reaction time in an internal mixer and along the screw axes with methyl methacrylate. The influence of residence time on the degree of grafting in an internal mixer and a twin-screw extruder was studied through measuring reaction yields with respect to reaction time in a mixer and evolution of reaction yield along the screw axis. The degree of grafting increased with initial monomer and peroxide concentration. The grafting reactions with three different peroxides were also investigated. The grafting levels were similar to maleic anhydride and suggested that only an individual methyl methacrylate unit be grafted. The melt viscosity was dramatically reduced with addition of peroxide. A kinetic scheme of our reaction system for methyl methacrylate was proposed and compared with the experimental results.