Provided are transition metal catalytic systems for preparing ethylene homopolymers or copolymers of ethylene with α-olefins. More specifically, provided are Group 4 transition metal catalysts, which is characterized in that the catalyst comprises around the Group 4 transition metal a cyclopentadiene derivative, and at least one aryloxide ligand(s) having a fluorenyl group or a derivative thereof (which is ready to be substituted at 9-position) that functions as an electron donor and serves to stabilize the catalytic system by surrounding an oxygen atom that links the ligand to the transition metal at ortho-position, and there is no cross-linkage between the ligands; catalytic systems comprising such transition metal catalyst and aluminoxane cocatalyst or boron compound cocatalyst; and processes for preparing ethylene homopolymers or copolymers of ethylene with α-olefins by using the same.