To obtain a composition excellent in workability which prevents viscous elevation of unvulcanized rubber as well as giving hardness and up-effect of equal or more regidity to a nylon-short fiber in less short amount by formulating a specific amount of a fibrilating polyvinyl alcoholic short fiber to a rubber component.

A rubber composition is formulated with 1-10 pts.wt. of a fibrilating polyvinyl alcoholic short fiber to 100 pts.wt. of a rubber component. The short fiber is not more than 5 μm in diameter, preferably not more than 1 μm and not more than approximately 5 mm in fiber length. The polyvinyl alcoholic short fiber easy of fibrilation, which is prepared by a mixed spinning using a solvent-wet-cold-gel spinning method, is kneaded with a diene rubber to give a master batch rubber dispersing the fibrilated short fiber. Further, the rubber composition is obtained by formulating and kneading the diene rubber as well as carbon black, a phenolic resin and an additive if necessary.