A MgCl2-supported catalyst containing diisobutyl phthalate (DIBP) and 2,4-pentadiol dibenzoate (PDDB) as internal donors was prepared. Propylene polymerizations were carried out using the catalyst in the absence or presence of an external donor. The resulting polymers were characterized by C-13-NMR, crystallization analysis fraction (CRYSTAF) and gel permeation chromatography (GPC). The performance of the catalyst was compared with that of other catalysts containing donor-free, DIBP and PDDB as internal donors respectively. The results demonstrated that the catalyst containing mixed internal donors not only had high activity and stereospecificity but also produced the polymer with relatively broad molecular weight distribution and the highest [mmmm] value. C-13-NMR analysis results indicated that strongly coordinating donors gave more stereoregular polymers, which was further confirmed by CRYSTAF data. The effects of mixed internal donors on the catalyst properties were discussed systematically. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 120: 36-42, 2011