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[Display omitted] • The morphology evolution of Hägg carbide Fe 5 C 2 under hydrogen conditions was explored. • Carbon-rich facets expose more area surface in Wulff construction with decreasing hydrogen chemical potential, while iron-rich facets are in contrast. • The reduction of surface carbon to methane for (1 1 1) and (11-1) facets is more facile than other facets. In heterogeneous catalysis, the interaction between adsorbates and substrate surface plays an important role on surface structure evolution and even the reaction mechanism. Herein, unified DFT calculation and ab initio atomistic thermodynamics were adopted to explore the interaction between hydrogen and multiple Fe 5 C 2 surfaces. The computation indicates that the dissociation of hydrogen molecule is both thermodynamically and kinetically favorable on Fe 5 C 2 surfaces. The adsorption energy of hydrogen atoms is related to the summed H bond valance (SBV H), a higher SBV H corresponding to a more stable adsorption strength. Based on calculated surface free energy with different hydrogen coverage, the equilibrium crystalline morphology evolution of Fe 5 C 2 under dynamic conditions was constructed with Wulff theorem. C-rich facets expose more area in Wulff construction with decreasing hydrogen chemical potential. The generation of carbon vacancy through reducing to methane has also been investigated, and the reduction of (1 1 1) and (11-1) facets is more facile than other facets. We anticipate this work could complement some knowledge to help understanding the surface structure and performance of iron carbide catalysts under experimental conditions. [ABSTRACT FROM AUTHOR]