The present invention relates to a novel ligand compound, a transition metal compound and a catalyst composition including the same. The novel ligand compound and the transition metal compound of the present invention may be useful as a catalyst of polymerization reaction for preparing an olefin-based polymer having a low density. In addition, an olefin polymer which is polymerized using the catalyst composition including the transition metal compound may be used for the manufacture of a product having low melt index (MI) and high molecular weight.