Provided are a novel transition metal complex where a monocyclopentadienyl ligand to which an amido group is introduced is coordinated, a catalyst composition including the same, and an olefin polymer using the catalyst composition. The transition metal complex has a pentagon ring structure having an amido group connected by a phenylene bridge in which a stable bond is formed in the vicinity of a metal site, and thus, a sterically hindered monomer can easily approach the transition metal complex. By using a catalyst composition including the transition metal complex, a linear low density polyolefin copolymer having a high molecular weight and a very low density polyolefin copolymer having a density of 0.910 g/cc or less can be produced in a polymerization of monomers having large steric hindrance. Further, the reactivity for the olefin monomer having large steric hindrance is excellent.