The present invention relates to a novel ligand compound, a transition metal compound, and a catalyst composition containing the compounds. The novel ligand compound and the transition metal compound of the present invention can be useful as a catalyst for polymerization in the preparation of an olefin-based polymer having a low density compared with that of a CGC catalyst. In addition, an olefin polymer polymerized by using a catalyst composition containing the transition metal compound enables the preparation of a high-molecular-weight product having a low melt index (MI).