The simulative accelerated creep test (ACT) on Gleeble thermal-mechanical simulator was developed as a response to an overall need of gaining in a short time useful physical data for estimation of long-term behaviour of materials exposed to operation at elevated temperatures. While most of the design of components and estimation of power plants lifetime is based on long-term creep data, which are generally available for the plate and pipe creep resisting steels, in case of the welded joints on these components such data are not always easily found and e.g. for the repair welds are very often not available. Next to its short duration, demanded from the accelerated creep testing procedure are the following conditions: - The basic temperature and applied strains in the ACT must prevent odd transformations like secondary dissolution of carbides or intensive formation of non-equilibrium phases. - The final deformation at fracture must be like at real creep – just a few pct in total. - The depletion of weld metal or steel matrix in alloying elements must be achieved similar to that of crept steels and the carbide phases at onset of cracks must not be different. To meet these requirements a low-cycle thermal-mechanical deformation-relaxation test was developed, during which the sub-structural processes characteristic of creep occurred much faster, transforming the initial microstructure of the creep resisting steels and welds to the near-to-equilibrium one in less than 30hours. The formation of dislocation configurations and the sequence of carbide precipitation and coagulation, as confirmed by TEM investigations, well resembled these of the conventional creep, so the testing procedure, hence called the simulative accelerated creep test – ACT, has been verified this way.