Provided are a novel transition metal complex where a monocyclopentadienyl ligand to which an amido or alcoxy group is introduced is coordinated, a method of synthesizing the same, and olefin polymerization using the transition metal complex. Compared to a conventional transition metal complex having a silicon bridge and an oxido ligand, the transition metal complex has a phenylene bridge, so that a monomer easily approaches the transition metal complex in terms of structure and a pentagon ring structure of the transition metal complex is stably maintained. The catalyst composition including the transition metal complex is used to synthesize a polyolefin copolymer having a very low density less than 0.910 g/cc.