A process for polymerizing propylene is disclosed. The process involves charging propylene and about 90 to 99% of an organoaluminum cocatalyst to a reactor and heating this mixture to at least about 50° C. This is followed by addition of a premix of 1 to 10% of the organoaluminum cocatalyst with a magnesium halide-supported Ziegler-Natta catalyst. The temperature of the reaction mixture is maintained to produce a propylene polymer. The process gives improved catalyst activity.