The effects of thermal aging on the microstructure and mechanical property changes in high Cr oxide dispersion strengthened (ODS) steels were investigated by using TEM, microhardness and small punch (SP) tests. The materials used were produced by varying Cr contents from 14 to 22 wt% but keeping yittria contents within 0.36-0.38 wt%. Specimens were thermally aged at temperatures between 430 and 475 degrees C up to 1000 h. SP-ductile to brittle transition temperature and microhardness of thermally aged ODS steels were significantly increased as a function of Cr contents, aging time and temperature. The main cause of embrittlement is the formation of a Cr-rich alpha' phase. The hardening appears thermally activated and the apparent activation energy is measured in the range of 214-240 kJ/mol. It is coherent with the activation energy for the diffusion of Cr or Fe in this type of alloys. (c) 2007 Elsevier B.V. All rights reserved.