PURPOSE: To enhance a monomer conversion ratio and graft efficiency by reacting a grafting monomer with a particulate olefin polymer substance, while or after treating the polymer substance with a chemical free radical polymn. initiator.

CONSTITUTION: One or more kind of 5-240 pph of a grafting monomer (C) capable of being polymerized by a free radical is added to a particulate olefin polymer substance (A) over a sufficient time imparting an adding speed not exceeding 4.5 pph/min in an arbitrary adding level at 60-125°C while or after the polymer substance is treated with 0.1-6.0 pph of a chemical free radical polymn. initiator (B) composed of an org. compd. of which the decomposition half-life at used temp. is 1-240 min in a non oxidative atmosphere at 60-125°C to be reacted with the polymer substance and, thereafter, the unreacted C-component is removed and the unreacted B-component is decomposed and the residual free radical in the grafted granular olefin polymer substance is deactivated to obtain the title copolymer.