Disclosed is a catalyst for polymerizing .alpha.-olefins which comprises(A) a composition obtained by co-comminuting(a) a magnesium halide,(b) an organic acid ester,(c) a halogenated aliphatic or alicyclic hydrocarbon,(d) at least one ingredient selected from the group consisting of aliphatic hydrocarbons, alicyclic hydrocarbons, aromatic hydrocarbons, halogenated aromatic hydrocarbons, liquid propylene oligomers and aromatic ethers, and(e) an aluminum halide optionally added thereto and then heat-treating the resulting mixture together with titanium tetrachloride;(B) an organic aluminum compound; and(C) an organic acid ester or a complex thereof with an aluminum halide.