A blend of a rubber and a rubber modifier, the rubber modifier comprising an ethylene/.alpha.-olefin copolymer rubber (A) composed of one or more copolymers selected from the group consisting of a copolymer of ethylene and an .alpha.-olefin having 3-20 carbon atoms and a copolymer of ethylene, an .alpha.-olefin having 3-20 carbon atoms and a non-conjugated polyene, the copolymer rubber (A) having the parameters: an ethylene/.alpha.-olefin mole ratio in the range of 51/49 to 87/13, an intrinsic viscosity [.eta.] determined in decahydronaphthalene at 135.degree. C. in the range of 0.8 to 2.5 dl/g and an iodine value in the range of 0 to 40.