Three kinds of MgCl2-supported trivalent titanocene catalyst (Cat. I: Cp2TiCl2AlCl2/MgCl2, Cat. 2: CpCp*TiCL/MgCl2, Cat. 3. Cp2TiCl/MgCl2) were prepared and tested for propylene polymerization. It was found that Cat. 1, combined with ordinary alkylaluminum as cocatalyst, produced PP containing 31.8 wt % of isotactic PP in fairly good yield. On the other hand, Cats. 2 and 3 hardly showed any activity. The effects of diisopropyldimethoxysilane (DIPDMS) on isospecificity of the Cat. 1 also were investigated. The isotactic index (I.I.) of PP was improved drastically by the addition of DIPDMS as external donor and reached the value as high as 98.4%, even in the absence of any internal donors.