The present invention relates to a transition metal complex and a transition metal catalyst composition comprising the same for the preparation of an ethylene homopolymer or a copolymer of ethylene and α-olefin. More particularly, it relates to a group IV transition metal catalyst having a cyclopentadiene derivative and at least one aryl oxide ligand in which an oxygen-containing heterocycle is fused at the ortho-position around a group IV transition metal, with no crosslinkage between the ligands, a catalyst composition comprising the transition metal catalyst and an aluminoxane cocatalyst or a boron compound cocatalyst, and a process for preparing an ethylene homopolymer or a copolymer of ethylene and α-olefin using the same.