Propene was polymerized over the MgCl2-supported TiCl4/dioctylphthalate catalyst in heptane. Polymer products obtained under different polymerization conditions were separated into isotactic and atactic polypropenes by the extraction of boiling n-heptane. The effects of polymerization time, cocatalyst type, cocatalyst/catalyst ratio, polymerization temperature, and external base/cocatalyst ratio on the isotactic triad of the isotactic portion of polypropene were investigated. 2,2,6,6-Tetramethyl piperidine (TMPIP), dimethoxy diphenyl silane (DMDPS), and t-butylmethyl ether (TBME) were employed as the external Lewis base. High concentrations of the first two bases caused a decrease in isotactic triads in the isotactic polymer, while TBME showed no significant effects. The difference can be attributed to the different roles these external bases play in polymerization.