The individual roles of three different types of external Lewis bases, namely t-butylmethyl ether (TBME), 2,2,6,6,-tetramethyl piperidine (TMPIP), and dimethoxydiphenyl silane (DMDPS), in propene polymerization using MgCl2/dioctylphthalate (DOP)/TiCl4 catalyst were studied. The role of TBME was found to be distinctly different from others. More active sites were believed to be activated in the presence of TBME, whereas the others seemed to play a role of converting atactic sites into isotactic ones. That difference made it possible to improve both catalyst activity and isotacticity by employing TBME together with one of the others as the external base pair. Due to the differences in the coordination strength of these bases with the catalyst, the order of addition of the two bases became important in achieving good results.