The influence of cocatalyst on the molecular weight and stereoregularity of stereoblock polypropylene (SB-PP) synthesized with bis(2-phenylindenyl)zirconium dichloride (2) catalyst system was investigated. The highest tacticity SB-PP was produced from the catalyst derived from 2 with water and pentafluorophenol modified isobutylmethylaluminoxane (MMAO) (W/PM-MMAO) whereas triphenylcarbenium tetrakis(pentafluoroplienyl)borate, [Ph3C] [B(C6F5)(4)] (3), activated 2 yielded the lowest tacticity SB-PP. In particular, the [mmmm] pentad content of SB-PP polymerized by 2 at 20 C drastically increased from 41 to 63% by changing cocatalyst from MMAO to W/PM-MMAO. When the solvent fractionation was carried out, the size of the amorphous fraction decreased, and the amount of higher crystalline material increased upon using W/PM-MMAO. Furthermore, from the C-13 NMR spectra of all fractions, it was revealed that there are no significant difference in the microstructure of each fractions of polymer produced by 2/MMAO and 2/W/PM-MMAO. These results show that only the content of isotactic segments increases, and higher tacticity SB-PP was obtained when W/PM-MMAO was employed as a cocatalyst. As a result of the structural change induced by the addition of water and pentafluorophenol to aluminoxane, we propose W/PM-MMAO prevents ligand oscillation and locks in the rac-like structure which is more stable than meso-like structure. Therefore, the stereoselectivity of SB-PP synthesized by 2/W/PM-MMAO increases. Furthermore, the reduction of amount of free trialkylaluminum using water and pentafluorophenol causes increasing productivity and the molecular weight.