A new procedure of X-ray line profile analysis based on the dislocation model of strain anisotropy has been applied. The dislocation densities and the subgrain size-distributions in a MANET steel are investigated in the normal condition and after cyclic deformation at room temperature and 250 degrees C. It has been found that the average dislocation density in the normal condition is rather high and does not change much after fatigue at room temperature. After fatigue at 250 degrees C, however, the dislocation density was found to increase considerably, probably due to the strong temperature dependence of the mobility of dislocation in a martensitic steel. Even though the subgrain size-distribution was found not to change much on fatigue at room temperature, it widened however after fatigue at 250 degrees C. (C) 2000 Elsevier Science B.V. All rights reserved.