Density functional theory (DFT) was applied to study nitrogen monoxide adsorption and reactivity on LiNbO3 surface, and the results were compared with the experimental ones for a single-crystal sample by X-ray photoelectron spectroscopy (XPS). The total density of state of the cluster model of LiNbO3 (Li15Nb4O22H8) well represented the experimental result. This agreement supports the accuracy of the procedure of this DFT computation. An unrestricted local density functional (LDF) geometry optimization of NO on two surface cluster models, Li2Nb2O10H6 and Li2Nb2O9H6, which had one or two vacant coordination sites of oxygen atoms around niobium, were converged well as NO3- and NO2-, respectively, by adding one electron. These results agreed well with the assignments of observed peak of the spectrum for the N Is level after the adsorption of NO at room temperature on every facial cut sample, x-, y-, and z-cut, with XPS. The coordinatively unsaturated surface lattice oxygen atoms reacted with NO, and these reacted oxygen atoms were introduced into the adsorbates and produced NO3- and NO2- as shown in both the results of the computed simulation and the spectrum for the N Is level of the exposed sample at room temperature. This explanation for the reactivity of coordinatively unsaturated surface lattice oxygen was confirmed also from the change of the spectrum for the O 1s level of every facial cut sample before and after NO exposure at room temperature.