The present invention relates to a transition metal complex useful as a transition metal catalyst in the preparation of an ethylene homopolymer or a copolymer of ethylene and an α-olefin, a catalyst composition comprising the same and a process of preparing an ethylene homopolymer or a copolymer of ethylene and an α-olefin using the same. More particularly, it relates to a transition metal complex having a cyclopentadiene derivative and at least one phenyl oxide ligand substituted at the 2-position of phenyl with, for example, a silyl group having a C1-C30 hydrocarbon group or a C1-C20 hydrocarbon group, around a group IV transition metal, with no crosslinkage between the ligands, a catalyst composition comprising the transition metal complex and a cocatalyst selected from the group consisting of an aluminoxane and a boron compound, and a process for preparing an ethylene homopolymer or a copolymer of ethylene and an α-olefin using the same.