PURPOSE: To obtain the title modified product excellent in draw formability, by reaction, under specified conditions, between ultra-high-molecular weight polyolefin, a fluidity improver, a specific monomer and a radical initiator.

CONSTITUTION: (A) An ultra-high-molecular weight polyolefin ≥5 (pref. 7-30)dl/g in intrinsic viscosity [η], (B) a fluidity improver (e.g., paraffin wax) with a melting point lower than that of said polyolefin, (C) a monomer having olefinic unsaturated bond (e.g., acrylic acid), and (D) a radical initiator (e.g., benzoyl peroxide) are mutually mixed using a high-speed rotational agitating mixer at temperatures higher than the melting point of the component B, but lower than the melting point of the component A followed by melt blending at temperatures higher than the melting point of the component A to effect reaction of the monomer C with the component A, thus obtaining the objective modified product.