This article demonstrates that the molecular weight of propylene homopolymer decreases with time, and that the molecular weight distribution (MWD) narrows when a highly active MgCl2-supported catalyst is used in a liquid pool polymerization at constant H, concentration and temperature. To track the change in molecular weight and its distribution during polymerization, small portions of home polymer samples were taken during the reaction. These samples were analyzed by Cross Fractionation Chromatograph (CFC), and the resulting data were treated with a three-site model. These analyses clearly showed that the high molecular weight fraction of the distribution decreases as a function of time. At the same time, the MWD narrows because the weight-average molecular weight decreases faster than the number-average molecular weight. A probable mechanism based on the reaction of an external donor with AIEt(3) is proposed to explain these phenomena.