Although Ziegler-Natta (Z-N) catalysts were first discovered in 1953, more than 90％ of the commercial manufacture of polyethylene and polypropylene is still based on modifica tions and improvements of the original Z-N system[1-3].In a recent work, Fan et al studied ethylene-1-hexene coolymerization with MgCl2-supported catalysts prepared by reacting TiCl3(OAr) (Ar =C6H5-, 2,6-Me2C6H3-, 2,6-iPr2C6H3-) with MgCl2[4] ,with special emphasis paid to the changes of copolymer chemical composition distribution.The cat alysts showed higher activity and comonomer incorporation rate as compared with a simple MgCl2/TiCl4 catalyst without organo-ligands.The influence of copolymerization conditions on the catalytic behavior of these catalysts will be reported in detail in a subsequent paper.Al though extensive experimental studies on heterogeneous Z-N catalysts have been made to un ravel the basic mechanisms in the catalytic process in the presence of TiCl3 (OR) as chain transfer agent, little efforts have been performed from a theoretical point of view to shed light on these points.In this contribution we try to fill this gap, and DFT studies were under taken in an attempt to model possible mechanisms for the active site by the insertion of ethylene into Ti-C bond, we followed the classical approach in which all the steps-including intermedi ates and transition states between reactants and products were investigated when TiCl3 (OAr) has different structure ligands.