Stacking fault energy is a fundamental material parameter in the discussion of plastic deformational mechanisms in metals. In this work, first-principles calculations were performed to investigate the effect of hydrogen atom and hydrogen filled vacancies on the stacking fault energy in fcc Fe, which is important for understanding the hydrogen embrittlement mechanism in austenite steel. In a perfect crystal, hydrogen atom can increase both the unstable and stable stacking fault energies, because of the bond between hydrogen atom and its nearest Fe atoms. The effect of a hydrogen atom on a stacking fault is short-ranged, covering only two atomic layers. It is suggested that the vacancies induced by hydrogen atoms are the main factor decreasing the stacking fault energy, instead of the hydrogen atoms in fcc Fe. Moreover, a hydrogen atom filled in a divacancy can help decrease the stacking fault energy barrier further, which in turn promotes stacking fault formation. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.