As a model of olefin polymerization by a Ziegler-Natta catalyst, the mechanism of the insertion reaction of ethylene into CH3TiCl4AlH2 has been studied by ab initio SCF methods, a many-body perturbation theory, and a localized molecular orbital (LMO) charge centroid analysis. The structures of the reactant, the intermediate, the transition state, and the product have been optimized with the RHF/3-21G method. The reaction is two-step process : the first forms ethylene-Tri pi complex with about 4 kcal/mol barrier, and the second step is the carbon-carbon bond formation (the insertion of ethylene into the Ti-C bond) through a pull-push mechanism. For the cocatalysis, the Al-Cl bonds alternation occurs at the second step. It is clarified that the cocatalysis play a role in facilitating the pull-push mechanism through the Al-Cl bonds alternation. The activation energy barrier in the ethylene insertion process for the model without the cocatalysis (AlH2Cl) is 20.5 kcal/mol higher than that for the model including the cocatalysis at the present calculation level.