Oxidation tests of austenitic steel Sanicro25 and TP347HFG were implemented at 600–700 °C in supercritical water under 25 MPa without dissolved oxygen. With an increase in temperature, the weight change of TP347HFG steel increases significantly while the weight change of Sanicro25 increases relatively slow. A double layer oxide scale formed on TP347HFG and Sanicro25 steel, which was composed of Fe‐rich oxide layer and a Cr‐rich inner oxide layer. Small amount of Cr2O3 was observed for TP347HFG but a continuous Cr2O3 formed at interface of oxide and matrix for Sanicro25 at 600 and 700 °C. Furthermore, Fe2O3 is detected at 600–700 °C for Sanicro25. The effect mechanism of temperature and the Cr content on oxidation rate and oxide composition were discussed.