A modified copolymer rubber obtained by kneading at 200-280°C
(A) at least one copolymer rubber material selected from the group consisting of an ethylene-α-olefin copolymer rubber having a number-average molecular weight of 10,000-100,000 and an ethylene content of 15-85% by weight and an ethylene-α-olefin-nonconjugated diene copolymer rubber having a number-average molecular weight of 10,000-100,000, an ethylene content of 15-85% by weight and a nonconjugated diene content of 3.0% by weight or less, (B) a free-radical initiator, (C) at least one vinyl aromatic monomer and (D) an unsaturated dicarboxylic anhydride, the bound vinyl aromatic monomer content and the bound unsaturated dicarboxylic anhydride content in the modified copolymer rubber being 0.1-5% by weight and 0.1-5% by weight, respectively, based on the weight of the copolymer rubber material and a process for producing the same are disclosed. This modified copolymer rubber obtained has so excellent properties such as a high bound unsaturated dicarboxylic anhydride content, a low gel formation property, a low tendency of increase of the Mooney viscosity [ML 1+4 (121°C)], and an excellent moldability and storage stability that it can be suitably used for thermoplastic elastomer, blending polymer, adhesive etc.