Eleven Cr ferritic/martensitic steels with improved high temperature properties (PNC-FMS) were thermally-aged and neutron-irradiated. To understand the thermal-aging and the irradiation effects on their microstructural developments, microstructural observation was made. From the detailed observations and microchemical analyses, the following points were obtained. (1) When irradiated at 400-450 °C to doses up to 94.6 dpa, both dislocation loops and cavities formed in the ferrite phase region. But void swelling was estimated as 0.05% at the most. (2) When irradiated at moderate temperatures of 500-600 °C, M23C6 and M6C grew and entirely covered the prior austenite grain (PAG) boundaries, suggesting that carbide precipitation on the PAG boundaries contributed to the extended time periods of recovery of martensite laths. (3) When irradiated at high temperatures above 650 °C, formation and growth of equi-axial grains occurred in addition to the complete recovery of martensite laths and the huge growth of carbides. © 2008 Elsevier B.V. All rights reserved.