The hydrogen effect on a X65 carbon steel was investigated by tensile tests under both ex-situ and in-situ hydrogen charging conditions. The fractured samples were characterized and compared using a combination of scanning electron microscopy, electron backscattering diffraction, and energy-dispersive spectroscopy. The work highlights that the in-situ hydrogen charging is a necessity for investigation of hydrogen detrimental effects on the studied material, where a pronounced reduction in fracture elongation, the evolution of secondary cracks on gauge surface, and the corresponding brittle fractography were thoroughly characterized after in-situ testing. The reason resides in the rapid hydrogen outgassing effect, which was proved by Fick's law-based diffusion models. Then the interrupted tensile tests were performed to track the crack initiation and propagation behavior. The results show that the majority of cracks initiated at the interfaces of MnS and Al2O3 inclusions or between inclusions and matrix, which attributes to the elevated stress concentration around the inclusions. Moreover, the cracks were found to propagate along the {110} slip planes. •Ex-situ and in-situ H charging method was compared on a X65 carbon steel.•In-situ H charging is more suitable for investigation of hydrogen embrittlement.•H-induced cracks initiate at non-metallic inclusions due to stress concentration.•H-induced cracks propagate in transgranular manner along the {110} planes.