This invention provides a process for preparing polyolefins by polymerizing or copolymerizing olefins using as the catalyst a titanium compound-containing solid component and an organometal compound, said solid component comprising a substance obtained by copulverizing (1) magnesium halide and/or manganese halide, (2) an organic halide compound, (3) a chain or cyclic silicone compound with a recurring structural unit represented by the general formula ##STR1## wherein R' and R" respectively represent a hydrogen, alkyl, aryl, alkoxy or hydroxyl group and (4) tetravalent and/or trivalent titanium compound(s).