© 2021 Hydrogen Energy Publications LLCCarcinogenic hexavalent Cr in the current iron oxide-based catalysts of high temperature water-gas shift (HT-WGS) reaction is great environmental concern. Interpreting the role of the Cr in this important industrial catalyst system is required. In the present study, we investigated substitution of Cr atoms into the most stable termination of Fe3O4 (111) slab surface by spin-polarized periodic DFT approach to comprehend the role of Cr. We applied the projector augmented-wave (PAW) method within the Perdew-Wang 1991 (PW91) form of the generalized gradient approximation (GGA) on the Vienna Ab-initio Simulation Package (VASP). The calculations point out that Cr atoms choose being below the surface FeO6 sites. Cr slightly affects the dissociative H2O adsorption. There is no effect on the CO adsorption. Oxygen vacancy is favored to form on the topmost layer with less vacancy formation energy. Substitution of Cr into the structure increases the oxygen vacancy formation energy. This indicates that Cr does not act as a chemical promoter and does not affect the catalytic activity positively, which is experimentally confirmed by the previous studies.