Two kinds of Cr25Ni20 austenitic heatresistant steels with different carbon content were heated to 800 and 1 100 ℃ respectively for 96 h. The cyclic oxidation curves were analyzed and the surface morphology of oxide films were observed by scan electron microscope. The composition of oxide film phase composition was analyzed by XRD method and the adhesion between oxidation film and metal substrate was measured through the scratching method. The results showed that the oxide film was made by diamond structure Cr2O3 grain and spinel structure of MnCr2O4 grain. With the increase of oxidation temperature, the oxide grains became bulky and transferred from the diamond structure into the spinel structure. The increase of carbon content caused the occurrence of "chromepoor" phenomenon, slowed down the formation of the protective oxide film and reduced the adhesive force. With the increase of oxidation temperature, the thickness of the cross section of oxide film increased and the damage degree of oxide film increased too.