The flow stress and microstructure evolution of 9Cr oxide dispersion strengthened (ODS) steels were investigated by isothermalcompression at 1000–1150 oC with strain rates from 0.001 to 1 s−1. Microstructure characteristics of ODS steelsunder various condition were studied using the EBSD and TEM techniques. At each deformation temperature, PortevinLe--Chatelier (PLC) effect characteristics of repeated serrated stress was observed, as the strain rates were 0.01, 0.1 and 1 s−1. The serrations fluctuated about the mean flow curve in quick succession, and the critical strain for the PLC effect was relatedto both the deformation temperature and strain rates. The stress drops at strain rates of 0.01 and 0.1 s−1 were smaller thanthose at 1 s−1. The strain rate affected the dynamic recovery and recrystallization of austenite grains, as well as the texturecomponents of martensite transformed from austenite. Cap-shaped oxide particles in as-fabricated and hot deformed steelswere characterized in detail. The cap parts of cap-shaped oxide particles were depleted of alloying elements, and the capswere crystal or amorphous. Large-sized oxide particles were easier to be deformed than small-sized ones. Despite of theoxide deformation, the PLC effect was still suggested to be related to the solute-dislocation interaction.