The ethylene/.alpha.-olefin/diene rubber composition of the present invention is a specific ethylene/.alpha.-olefin/diene rubber composition comprising a high-molecular ethylene/.alpha.-olefin/diene copolymer rubber (A) having a specific molecular weight distribution, a specific intrinsic viscosity [.eta.] and a specific iodine value, and a low-molecular liquid ethylene/.alpha.-olefin copolymer (B) having a specific intrinsic viscosity [.eta.]. This ethylene/.alpha.-olefin/diene rubber composition of the invention is excellent in both the processability (flowability) and the shape retention properties in the vulcanization molding process, and the rubber composition can provide a vulcanized rubber molded product which is excellent in surface smoothness, mechanical strength, low-compression permanent strain properties, resistance to abrasion and resistance to dynamic fatigue.