Publisher Summary The recent discovery of silane compounds is especially noteworthy for the improvement of isotacticity. The effects of alkoxysilane compounds on isotacticity have been studied for both Ticl3 and MgCl2-supported type catalyst, but unfortunately, the correlation between isotacticity and the type of silane compound has not been made clear. The object of this chapter is to demonstrate the effect of structural and electronic factors of alkoxysilane compounds on the activity and isotacticity of propene polymerization. The volume of the alkoxysilane compound is considered as the structural factor, and the electron density of oxygen atom in alkoxy group is assumed as the electronic factor as the alkoxysilane compounds interact with Ti in the catalyst at the oxygen atom. For quantitative analysis, the volume and electron density are determined by molecular orbital (MO) calculations. In the chapter, the SiO2-supported catalyst is used for the evaluation of the additive effects of alkoxysilane compounds on isotacticity. The additive effects of dialkyldimethoxysilane compounds on both activity and isotacticity have been studied with SiO2-supported catalyst. Both, activity and isotacticity change with the type of silane compound. The application of MO calculation to the silane compounds makes possible quantitative analysis of changes in activities and isotacticities. Isotactic polymerization rate depends on the volume of silane compound while the atactic polymerization rate is related to the electron density of the oxygen atom in silane compounds.