A process for producing a partially or completely crosslinked olefinic rubber composition which comprises (A) 1 to 99 parts by weight of an ethylene-.alpha.-olefin copolymer having ethylene units and C.sub.3-C.sub.20 .alpha.-olefin units, and (B) 1 to 99 parts by weight of an olefinic resin, total amount of (A) and (B) being 100 parts by weight, said (A) having a degree of crosslinking of 50% or more and a degree of swelling of 5 to 40.