Martensitic stainless steel 17-4PH is a precipitation hardening material with a combination of high strength and good ductility and has been widely used in various modern industries. However, it is also susceptible to hydrogen embrittlement in some environments. In this paper, susceptibility to hydrogen embrittlement in this material with different conditions has been studied using a slow strain rate tensile test with an in-situ electrochemical hydrogen charging. By fracture and microstructure analysis, the hydrogen embrittlement mechanisms have been evaluated. It was found that the 17-4PH steel in the solution annealed condition is susceptible to hydrogen embrittlement. Hydrogen-enhanced decohesion (HEDE) is the main hydrogen embrittlement mechanism. Tempering at 510 degrees C reduces susceptibility to hydrogen embrittlement by precipitation of different nano precipitates, mainly nano Cu rich phase and nano austenitic phase. This also leads to a transition of hydrogen embrittlement mechanism from HEDE to HELP (hydrogen enhanced localized plasticity). A synergetic effect of HELP and HEDE has been observed. This work increases the knowledge to reduce and finally avoid susceptibility to hydrogen embrittlement in 17-4PH steel material.