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Prediction of residual lifetime of the power plants and petroleum refineryobjects is a very important problem. For its solution it is necessary to taking into accountmaterial degradation during service. There is an opinion, that hydrogen that dischargesfrom the metal surface during manufacturing, is one of the most important factors of thedegradation process. Hence, the important role in residual life prediction is attributed tothe development of in-laboratory methods for simulation of in-service degradation ofmaterials.The aim of this work is to investigate the effect of the exposure specimens to gaseoushydrogen on the change of the microstructure and threshold parameters of fatigue crackgrowth in 15Kh2MFA steel.Two methods of the in-laboratory ageing in hydrogen environment weredescribed. In the first one the beam specimen was subjected to the long-term uniaxialtension in hydrogen environment at temperature 450 oC. In the second method the beamspecimen was subjected to thermocycling in hydrogen. The thermocycling itself does notcause any essential internal stresses in the unrestrained specimen.It was shown that the effective threshold parameter is sensitive to the changes ofmicrostructure in metal after isothermal holding and thermocycling in hydrogen. Thesechanges depend on stress level during isothermal holding and a number of thermocycles inhydrogen. It was concluded that just hydrogen is responsible for the acceleration ofdiffusion process, which is the necessary condition for the observed redistribution ofcarbides in such ageing conditions.