PURPOSE: To obtain an ethylene polymerization catalyst which has activity at least equivalent to that attained when an accelerator is used by mixing a reaction product of a V compound with a specified electron donor with an alkylhalosilane, a silica support and a specified Al promoter.

CONSTITUTION: This catalyst is obtained by mixing a V compound (e.g. vanadium trihalide) and an electron donor in the form of a liquid or an organic Lewis acid in which the V compound is soluble (e.g. alkyl aliphatic or aromatic carboxylate) (A) with an alkylhalosilane (B) represented by the formula (wherein R is a 1-10C alkyl; X is a halogen; (a) is 1 or 2; (b) is 2 or 3; and a+b=4 (e.g. methyltrichlorosilane), a silica support (C) impregnated with components A and B and a hydrocarbylaluminum promoter (D). This has activity at least equivalent to that attained when an accelerator is used.