In this paper we will try to further clarify the creep-strength degradation of selected advanced creep resistant steels. In order to accelerate some microstructural changes and thus to simulate degradation processes in long-term service, isothermal ageing at 650°C for 10000 h was applied to P91, P92 and P23 steels in their as-received states. The accelerated tensile creep tests were performed at temperature 600°C in argon atmosphere on all steels in the as-received state and after long-term isothermal ageing, in an effort to obtain a more complete description of the role of microstructural stability in high temperature creep of these steels. Creep tests were followed by microstructural investigations by means of both transmission and scanning electron microscopy and by the thermodynamic calculations. It is suggested that microstructural instability is the main detrimental process in the long-term degradation of the creep rupture strength of these steels.