[en] Highlights: • A highly hydrogen trapping capability was determined on the C atoms in ZrC. • A new diffusion path of hydrogen and its diffusion coefficient in ZrC bulk was predicted. • The effect of hydrogen interstitial defect on the elastic properties of ZrC was investigated. - Abstract: The structure stability, diffusion behavior of interstitial hydrogen, and its influence on the elastic properties of stoichiometric ZrC bulk are investigated by first-principles calculations. The interstitial hydrogen in stoichiometric ZrC bulk prefers to be trapped by the C atom with C-H distance of 1.15 Å, and its formation energy is 1.30 eV after ZPE correction. The interstitial hydrogen promotes the formation of Zr/C vacancy in ZrC bulk due to the electron localization effects and the effect on the C vacancy is more significant. A new energetically favorable diffusion path of hydrogen in ZrC bulk is predicted as from one center C-H site in the Zr/C cube to the other neighboring one by climbing the metastable Zr/C cubic center site with an energy barrier of 0.64 eV, and then followed by twice rotations around the nearest neighboring C atom with an energy barrier of 0.26 eV. A high hydrogen resistance can be expected on the ZrC ceramic due to the extremely low hydrogen diffusivity and the insignificant effects of hydrogen on the elastic properties.