The work aims to study the effect of Nb content on the steam oxidation behavior of austenitic heat-resistant steel in high temperature environment.Two HR3C austenitic heat-resistant steels with different composition were selected and subjected to oxidation experiments in a pure water vapor environment at 700 ℃ and 25 MPa for 1000 h. Scanning electron microscopy, X-ray diffraction instrument and optical microscopy technology were used for comparative studying on high temperature oxidation kinetics, microstructure evolution and the composition constituent of the oxides and alloys, to observe and investigate the effect of Nb on the steam oxidation behavior of HR3C austenitic heat-resistant steels. The content of Nb had a significant effect on the steam oxidation performance of HR3C austenitic heat-resistant steel. The oxidation weight gain of HR3C-0# alloy was 0.7746 mg/cm2, the microhardness was 258.9HV. And the weight gain of HR3C-1# alloy was 1.716 19 mg/cm2, the microhardness was 201.4HV. Grain refinement occurred in both alloys during the steam oxidation process. When the content of Nb is in a relative lower range, the HR3C austenitic steel exhibits good resistance to steam oxidation and high hardness. On the other hand, the steam oxidation resistance and microhardness of the alloy is deteriorated as the content of Nb in the alloy is over high. A large amount of unprotected oxide-island Fe2O3 formed on the surface of the sample, accompanied by the appearance of grain refinement.