Abstract: One shot mouldable and curable homogeneous rubber composition consisting mainly of polyisoprene rubber and a monomer mixture composed of divinylbenzene and .alpha.-methylstyrene in specifically limited weight ratios of the rubber/the monomer mixture and of divinylbenzene/.alpha.-methylstyrene and containing small amounts of organic peroxide and at least one of amine compound and phenol compound. The cured product of the rubber composition has high Young's modulus, tear energy and tensile strength retention. More improved cured rubbers having these physical properties well balanced in a high level can be easily obtained by additionally adding a small amount of sulfur or p-quinonedioxime to the above rubber composition.