To provide a graft copolymer suitable as a dispersive viscosity index improver in a lubricant oil composition for an EGR engine.

The graft copolymer especially useful as the dispersive viscosity index improver in the lubricant oil composition has a polymer skeleton selected from the group consisting of an olefinic polymer, dienepolymer, vinyl polymer and vinylidene polymer and is grafted with an amine selected from a reaction product or the like of N-p-diphenylamine, 1, 2, 3, 6-tetrahydrophtalimide, 4-anilinophenylmethacrylamide, 4-anilinophenylmaleimide, 4-aninophenylitaconamide, acrylate or methacrylate of 4-hydroxydiphenylamine, p-aminodiphenylamine or p-alkylaminodiphenylamine with glycidyl methacrylate and a mixture thereof, and the graft copolymer is manufactured using a graft condition in solution or bulk in a single step.