First principles calculation reveals that the Nb16H and Nb15WH phases with H at tetrahedral(T) and octahedral(O) sites are energetically favorable with negative heats of formation (Delta H-f), while the Nb15WH(T) phase has higher Delta H-f value than the Nb16H(T) phase, suggesting that the addition of W would decrease the solubility of H in Nb. Calculation also shows that the addition of W should have an important effect on diffusion behavior of H in Nb, i.e., the energy barrier of H diffusion becomes lower, and more diffusion paths are introduced, which would fundamentally increase the diffusion coefficient and permeability of H in Nb15W. The calculated results are in good agreement with experimental observations in the literature, and could provide a deep understanding to the effects of W on various properties of NbH phases. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.