The water-containing atmosphere plays an important role in the improvement of proton conductivity of LaNbO4. The interaction between the water molecule and the oxygen-deficient (010) surface of LaNbO4 crystal has been investigated via first-principles calculations. The water molecule is set at different heights over the oxygen vacancy and the exposed niobium atom in Nb-O tetrahedron. The adsorption and dissociation behavior of the water molecule to the surface are illustrated and analyzed by the total and localized density of states (DOS) plots. By finely adjusting the heights of the water molecule from the surface, the relatively stable position for the water molecule is determined by free energies of the hydrated slabs. The water molecule prefers to adsorb onto LaNbO4 (010) surface and then to dissociate into one proton and one hydroxyl over the oxygen vacancy rather than over the exposed niobium atom in Nb-O tetrahedron. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.