Abstract: A highly crystalline propylene polymer composition which is obtained by preliminarily polymerizing, in the presence of a catalyst comprising a solid titanium trichloride catalyst complex containing a complexing agent and having an aluminum content of at most 0.15 by atomic ratio of aluminum to titanium and an organoaluminum compound and optionally an electron donative compound, 3-methylbutene-1 in an amount exceeding 10 g per g of the solid titanium trichloride catalyst complex and then polymerizing propylene, or propylene and other .alpha.-olefin having from 2 to 18 carbon atoms, in an amount of at least 500 g per g of the solid titanium trichloride catalyst complex, and which contains from 0.01 to 20% by weight of the 3-methylbutene-1 polymer based on the total weight of the 3-methylbutene-1 polymer-containing propylene polymer composition.