Objective To study the effect of pre-existed hydrogen in specimen on the mechanical properties. Methods For a type of solution-treated and three types of heat-treated 18Ni maraging steels, electrochemical permeation was employed to determine effective hydrogen diffusion coefficient (Deff); thermal desorption analysis (TDA) was carried out to assess activity energies (Ea); slow strain rate tensile tests were performed to characterize the mechanical properties and further evaluate the hydrogen embrittlement (HE) susceptibility of pre-charged and cadmium coated specimens in comparison of uncharged specimens. Results The results showed that Deff of the solution-treated specimen was 1.40×10?8 cm2/s, which was the largest among four specimens. Deff of three heat-treated specimens gradually decreased with the rise of aging temperature, which were 6.23×10?9 cm2/s for 465 ℃, 5.52×10?9 cm2/s for 490 ℃ and 2.84×10?9 cm2/s for 530 ℃ respectively; whereas, Ea gradually rose. Four specimens all exhibited HE susceptibility, which increased with the rise of charging current density. The HE susceptibility indexes for T465 and T490 were both higher than 58%, while that for T530 was smaller than 40%. In the fracture morphology of the four specimens, cracks initiated from the centre and radiated to surrounding. Centre crack source was a typical intergranular feature, while the extension area was quasi-cleavage cracking. Conclusion The hydrogen embrittlement of overaging samples is the best. The pre-existed hydrogen in the sample gathers in the center during tensile test, which causes intergranular crack in the center and reverse to dynamic hydrogen charging tensile fracture.