Provided are a novel transition metal complex where a monocyclopentadienyl ligand to which an amine-based group is introduced is coordinated, a method of synthesizing the same, and olefin polymerization using the transition metal complex In the novel transition metal complex, an imino phenyl group is not cross-linked to a metal atom and directly introduced to a cyclopentadiene (Cp) ring. The catalyst composition including the transition metal compound is used to obtain a polyolefin copolymer having a very low density less than 0.910 g/cc.