Provided are ligand compounds selected from among N-(diphenylphosphino)-1,1-diphenyl-N-(4-phenylbutan-2-yl)phosphinamine and N4,N4-bis(diphenylphosphino)-N1,N1-diethylpentane-1,4-diamine, a catalyst system for olefin oligomerization, and a method for olefin oligomerization using the same. The catalyst system for olefin oligomerization has excellent catalytic activity, and yet, exhibits high selectivity to 1-hexene or 1-octene, thus enabling more efficient preparation of alpha-olefin.