Hydrogen embrittlement of a nickel-based superalloy IN718 was investigated using slow strain rate tensile tests. Post-mortem observation of fractured samples was performed to explore hydrogen-assisted failure mechanisms of the alloy. The results reveal that hydrogen charging reduces yield strength, tensile strength, fracture strain and work hardening rate. With increasing current density, yield strength and tensile strength reduce linearly and fracture strain decreases exponentially. Furthermore, the crack initiation and propagation in hydrogen-charged region depends on the distribution of delta phase in the alloy. For needle-shaped delta phase within the grains, the nucleation of voids takes place at the intersections between dislocation slip bands and delta phase due to the hydrogen enhanced localized plasticity (HELP)-assisted shear localization and possible hydrogen agglomeration. For delta phase along the grain boundaries, the impingement of slip bands and local hydrogen accumulation at gamma-matrix/delta phase interfaces as well as hydrogen-enhanced decohesion (HEDE)-assisted decohesion lead to the void nucleation at the interfaces. Because of the decoration of delta phase at the grain boundaries, hydrogen-assisted cracking preferentially propagates along the grain boundaries. It is hence suggested that the synergistic interplay of HELP mechanism and HEDE mechanism can be used to explain the embrittlement of the alloy. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.