Microcrystallization is an important way to improve the oxidation resistance of alloy at high temperature. In this paper, cast Fe-Cr alloys with different Cr contents were treated by electrospark deposition to get microcrystalline structure. The oxidation behavior of as-cast and microcrystalline Fe-Cr alloys at 900 ℃ in air was studied. Oxidation kinetics curves, phase analysis and surface morphology results show that the diffusion rate of chromium is enhanced by microcrystallization treatment when Cr content is low. Yet the content of Cr is not enough to form continuous oxide film so that the oxidation resistance of Fe-5Cr alloy was decreased. The critical Cr content to form protective oxide scale was reduced by microcrystallization treatment. The oxidation resistance of microcrystalline Fe-9Cr and Fe-13Cr alloys was obviously improved. The high temperature oxidation resistance of microcrystalline Fe-13Cr alloy is the best, which is due to the formation of continuous dense protective oxide scale on the surface after microcrystallization treatment