In a method for producing olefin polymers which comprises polymerizing ethylene monomer alone, or two components comprising ethylene and .alpha.-olefins of the formula, CH.sub.2 .dbd.CHR (in which R is a straight or branched alkyl group having 1 to 6 carbon atoms), or three components comprising ethylene, said .alpha.-olefin and one or more of straight or cyclic diene or polyene monomers having a non-conjugated double bond, in the presence of a catalyst, the improvement which comprises using a catalyst system comprising (1) an organo-aluminum compoum, (2) a 3- to 5-valent vanadium compound soluble in organic inert solvents, and (3) (A) a compound of the formula, ##STR1## (in which R' is a group having a straight or cyclic ether linkage, and X is a halogen atom), or (B) a reaction product of phosphorus pentachloride with an alcohol as an activating agent.