Effects of an external donor on propylene polymerization were investigated for Mg(OC₂H₅)₂-based Ziegler-Natta catalysts. Stopped-flow technique was applied to this study to observe polymerization behavior more precisely. It was confirmed that the presence of alkoxysilane external donors induced the increase of active sites concentration resulting in activity increase. Furthermore, it was postulated that even though desorption of the internal donor from the catalyst rarely happened during the stopped-flow polymerization because of its extremely short polymerization time, the external donors, Csi and Dsi can interact with the catalyst and affect the concentration of active sites.