The effect of different external silane donors on the activity and isotacticity of polypropylene prepared by using Mg(OEt)(2)-phthalate ester-TiCl4-AlEt3-alkoxy disilane catalyst systems has been investigated. In the case of catalyst systems containing (trimethylsilyl)methylalkyldimethoxysilanes [Me3SiCH2Si(OMe)(2)R] as external donors, the bulky Me3SiCH2-group was effective in converting aspecific sites into isospecific ones, followed by the increase of activity and isospecificity of the obtained polymer. Catalyst activity and isotacticity of the polymer increased with the decrease of the number of alkoxy groups in the substituted alkoxy disilanes. The effect of alkoxy groups between alkoxy disilanes and alkylaluminum as cocatalyst on the active sites of the catalyst and the influence of the size of alkoxy groups in disilane compounds were examined, respectively. Some correlation between molecular weight, molecular weight distribution, and isospecificity was also observed.