PURPOSE: The titled composition, containing an olefinic (co)polymer and a specific diallyl isocyanurate, capable of uniform kneading and crosslinking, and having improved crosslinking efficiency due to a high crosslinking rate.

CONSTITUTION: A composition obtained by incorporating (A) an olefinic (co) polymer with (B) a 5-substituted-1,3-diallyl isocyanurate expressed by the formula (R is ≥14C alkyl group), e.g. 5-tetradecyl-1,3-diallyl isocyanurate, in an amount of preferably 2W15pts.wt. based on 100pts.wt. component (A). The resultant composition is then crosslinked and polymerized by irradiation with electron rays, etc. to give the aimed olefinic polymer. The component (B) is preferably obtained by reacting diallyl isocyanurate in an aprotic polar solvent in the presence of a basic substance at 60W150°C.