The apparent activation energy for rupture life sometimes changes from a high value of short term creep to a low value of long term creep. This change results in overestimation of rupture life recognized recently in advanced high Cr ferritic steels. The present study examined how to detect the decrease of activation energy in 9-12 %Cr steels with tempered martensitic lath microstructure. During aging without stress hardness of the tempered martensite microstructures remains almost constant in short term, whereas it decreases with increasing time after long term exposure. The onset of hardness drop can be a good measure of the decrease of activation energy. Causes of the hardness drop and the decrease of activation energy are discussed.