PURPOSE: To obtain a high crosslinked nitrile rubber composition having excellent properties for seals and gaskets and being capable of being blended with a polyvinyl chloride by using a specified highly crosslinked nitrile rubber.

CONSTITUTION: A highly crosslinked nitrile rubber (A) having repeating units derived from 1,3-butadiene, acrylonitrile, and a crosslinking agent and having a Moony viscosity of about 50-120, a swelling index of below 10%, a mill shrinkage of below 10%, and a gel content of above 90% is blended with 1-30 phr of a plasticizer (B) to obtain a nitrile rubber composition. This composition is useful to fabricate seals and gaskets in automobiles and construction applications, and about 5 to 300 pts.wt. this composition is mixed with 100 pts.wt. polyvinyl chloride, about 20 to 120 pts.wt. plasticizer, and about 1 to 3 pts.wt. stabilizer to obtain a rubber composition useful for seals for automobile bodies.