To obtain a rubber composition being capable of forming a vulcanizate proof against the formation of water otherwise formed by the secondary decomposition of the constituent organic peroxide and therefore proof against deterioration in electrical properties by compounding a rubber with an organic peroxide and tetraethylenepentamine.

The amount of the organic peroxide added is desirably about 1-4 pts.wt. per 100 pts.wt. rubber, and the amount of the tetraethylenepentamine added is desirably 10-30,000 ppm based on the entire rubber composition. The added tetraethylenepentamine inhibits the reaction in which the alcohol as a decomposition product formed from the organic peroxide during crosslinking is secondarily decomposed to form water when heated in the subsequent step and enables the crosslinked rubber molding to have a low water content. The molding can therefore be reduced in deterioration in electrical insulation properties, dielectric properties, etc. It is desirable that the rubber composition is used as a covering material such as an insulator or a sheath. It is desirable that it is used as an insulating material for a connecting part of an electrical cable.