Positron annihilation, microstructural and hardness studies were carried out in a 9Cr reduced activation ferrtic ‐ martensitic steel (9%Cr‐1%W‐0.1%C) subjected to isochronal annealing in the temperature range of 300 to 1273 K. The measured lifetime in the ‘as‐received’ (normalized and tempered) condition was 147 ps, which exhibited considerable reduction around 670 K, indicating the annealing out of small vacancy‐defects in the matrix or matrix‐precipitate interfaces. The observed positron lifetime exhibited a broad maximum between 670 K and 1070 K, indicating the existence of second annealing stage. Between 1070 K and 1270 K, a sharp maxium is exhibited in lifetime as third annealing stage. Doppler S‐parameter has also exhibited similar annealing stages. Ab‐intio positron lifetime calculations for various defect‐impurity cluster configurations in ferritic‐matrix based Fe system are reported to understand the experimental results. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)