A spin-polarized density functional theory calculation was carried out to study the adsorption of NHx species (x = 1-3) on a TiO2 anatase (101) surface with and without hydroxyl groups by using first-principles calculations. It was found that the present hydroxyl group has the effect of significantly enhancing the adsorption of monodentate adsorbates H2N-Ti(a) compared to that on a bare surface. The nature of the interaction between the adsorbate (NHx) and the hydroxylated or bare surface was analyzed by the Mulliken charge and density of states (DOS) calculations. This facilitation of NH2 is caused by the donation of coadsorbed H filling the nonbonding orbital of NH2, resulting in an electron gain in NH2 from the bonding. In addition, the tipper valence band, which originally consisted of the mixing of O 2p and Ti 3d orbitals, has been broadened by the two adjacent H 1s and NH2 o(y)(b) orbitals joined to the bottom of the original TiO2 valence band. The results are important to understand the OH effect in heterogeneous catalysis.