Organic micro-fiber reinforced rubber compositions having high elastic modulus, excellent crack growth resistance, impact cut resistance and rebound resilience and a small swell value are prepared by incorporating 10-100 parts by weight of carbon black, 3-30 parts by weight of organic micro-fibers based on 100 parts by weight of at least one rubber selected from a group consisting of natural rubber, synthetic polyisoprene rubber, butyl rubber, halogenated butyl rubber, polybutadiene rubber, styrene-butadiene copolymeric rubber, ethylene-propylene-diene ternary copolymeric rubber and acrylonitrile-butadiene copolymeric rubber, said micro-organic staple fibers being lower than 30.degree. C. or higher than 120.degree. C. in glass transition temperature of amorphous portion and higher than 160.degree. C. in melting point of crystal portion, and having an average length of the micro-fibers of 0.8-30 .mu.m, an average diameter of the micro-fibers of 0.02-0.8 .mu.m and a ratio of the average length of the micro-fibers to the average diameter of the micro-fibers of 8-400.