The present invention provides a novel transition metal complex where a monocy-clopentadienyl ligand to which an amido group is introduced is coordinated, a method for synthesizing the complex, and olefin polymerization using the same. The method for preparing a transition metal complex according to the present invention comprises a step of blocking a by-reaction of a nitrogen atom using a compound containing a protecting group, and thus it is possible to prepare a transition metal complex in a simpler manner in a high yield. Further, the transition metal complex according to the present invention 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, sterically monomers can easily approach the transition metal complex.