The insertion mechanism of styrene polymerization with a Cp-based ansa-matallocene catalyst (Cp = eta(5)-C5H5) was investigated by the density functional theory (DFT). The metallocene catalyst adopted in this study, Si(CH3)(2)CP2Ti(CH3)(2), is experimentally known as a highly syndiospecific catalyst for styrene polymerization. A cationic species, SiH2CP2Ti1-CH3, is used as an activated form of the catalyst for modeling the initiation of styrene polymerization when one calculates the first insertion of a styrene monomer into the cationic species. Calculations for the second insertion of styrene monomer into the product of the first insertion were also performed. The results of the second insertion of styrene monomer suggest that the frontside insertion of the second styrene leads to a Hp-transfer reaction, while the backside insertion, which is favored over the frontside insertion, produces a highly syndiotactic product without the Hp-transfer reaction. This is in agreement with the experimental result. In the backside styrene insertion, the transition state is characterized by a beta-agostic interaction than an alpha-agostic interaction because the alpha-agostic interaction has the repulsive interactions between catalytic ligands and substituents bonded to C, of growing polymer chain. The stereoselectivity of the backside insertion is rationalized by the chain-end-control mechanism, and this mechanism also rationalizes higher insertion steps of styrene monomers.