Abstract The H–Nb system has been thermodynamically modeled by combining existing available experimental data on phase boundaries, invariant points and hydrogen vapor pressure of the system. The phase diagram and phase transitions at high pressure were considered. The gas phase was treated as ideal mixture, the liquid phase was modeled by substitution solution model and the solid phases were described by sublattice model. The present calculations show satisfactory agreement with most experimental data. According to the calculated results, δ phase cannot exist steadily above 297 K at normal pressure and precipitates ahead of β phase above 8.5 MPa.