The electronic factors of external alkoxysilane donors used in modern Ziegler-Natta catalyst systems were estimated by quantum mechanical MNDO (modified neglect of diatomic overlap) calculations on model compounds. The calculated electron densities of oxygen atoms in alkoxy groups were plotted against the experimental activity and the isotacticity of propene polymerization. The steric hindrance and the volume of the compounds were investigated as structural factors important to the polymerization. The results indicate that the performance of the external alkoxysilane donor is affected by both electronic and structural factors. The average electron density around the electron-donating oxygens has some influence on the deactivating power of the donor, but when the hydrocarbon part is sterically large the sterical effects apparently dominate.