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