The present paper deals with hydrogen embrittlement (HE) susceptibility of a weld thermal simulated heat affected zone of X70 structural steel in high pressure hydrogen gas at 20 °C. Fracture mechanics Single Edge Notched Tension tests at various hydrogen pressures (0.1, 0.6, 10 and 40 MPa H2) have been carried out. The HE susceptibility was quantified through the measurement of the fracture toughness KQ and J (the effect of hydrogen pressure was addressed through linear load increase conditions till failure was obtained). The results show that hydrogen causes a strong decrease in the fracture toughness with increasing hydrogen pressure. The critical hydrogen pressure for the onset of HE was observed to fall between 0.1 MPa and 0.6 MPa. These results were supported by Scanning Electron Microscope (SEM) investigations of the fracture surfaces which showed a clear shift in the fracture mode at 0.6 MPa H2. Moreover, constant load tests were carried out in order to investigate the influence of hydrogen exposure time. The results imply that fracture always occurs within 8 h and that longer time to failure is related to stronger toughness reduction. This effect is more pronounced for test at 40 MPa than at 0.6 MPa hydrogen pressure levels. 3D Finite Element calculations of hydrogen diffusion have been performed and the results are discussed in relation to the experiments, in order to attempt to identify the hydrogen populations (diffusible or trapped) which act predominantly on the embrittlement of the material.