Two unbridged metallocene catalysts, bis(2,4,7-trimethylindenyl)zirconium dichloride (met-l) and bis(2,4,6-trimethylindenyl)zirconium dichloride (met-II), which are different in the position of substituents on the six-membered ring of the indenyl ligands were synthesized. The effect of substituents in the two metallocenes on the propylene polymerization was studied in the presence of methylaluminoxane (MAO) and triisobutylaluminium (TIBA). From the analysis of microstructure determined by 13C-NMR, it was demonstrated that the polymers produced by met-II have higher [mmmm]isotactic sequences than that of met-I. Using a mechanism based on model statistical analysis, it was found that chain-end model was dominant for met-I. However, met-II obeys the concurrent two-sites model during polymerization, which can be attributed to the existence of"racemic-like" conformer in its system.``