is the amount effective for hardening of the rubber by means of crosslinkage and polymerization of the monomers the polyolefin rubber composition of the present invention comprises the above mentioned components however in addition an antioxidant an anti weathering agent a pigment a viscosity adjusting agent an inorganic additive and a fiber reinforcing agent can be blended with them moreover another vinyl polymer a synthetic resin elastomer natural rubber and the like can be blended with them as long as they don 39 t detract the physical properties of the polyolefin rubber this polyolefin rubber composition is then blended and a denaturated polyolefin rubber can be manufactured by subjecting the mixture to heat treatment when the polyolefin rubber composition of the present invention is heated the organic peroxide decomposes and acts as a radical polymerization initiator the mono olefin monomer and the polyolefin polyfunctional monomer begin to polymerize but because the polyfunctional monomer is present crosslinking occurs at the same time because hydrogen is split off from the molecular chain of the polyolefin rubber polymer a graft polymerization is also started and it is believed that the denaturated polyolefin rubber which is denaturated by the mono and polyolefin is formed when the denaturated polyolefin rubber is produced mixing can be performed using a commonly known mixing means such as a mixing roll a banbury mixer and the like a temperature close to ambient temperature is desirable as the mixing temperature the heating can be performed at the same time as the mixing or can be performed consecutively while heating extruding can be carried out in.