Five new alkoxysilanes with different sizes of hydrocarbon substituents were first synthesized and employed as external donors for propylene polymerization with a Ziegler-Natta catalyst. The effects of these and industrial alkoxysilanes with different sizes of hydrocarbon substituents on the microstructure of polypropylene were studied by SSA and C-13 NMR. The results showed that isotactic sequence length of polypropylene increased with the size of R-2 substituent on alkoxysilanes in the five donor systems, similar to the regularity of molecular weight, isotacticity, and thermal properties of polypropylene. Although the polypropylene produced by double cycloalkane substituents on alkoxysilanes had lower lamellae thickness L-1, its contents were the highest. Moreover, excess large volume of hydrocarbon substituents on alkoxysilanes might be detrimental to improving isotactic sequence length of polypropylene. Polypropylene prepared by MIPDMS had a more uniform distribution of the stereodefects, indicating that MIPDMS might be used for the industrial production of BOPP.