Isothermal oxidation was carried out on Fe and Fe-(1.0 and 5.0mass%)Cr alloy at 800°C and 1000°C in dry (PH2O=1x10-4 atm) and wet (PH2O=0.035 atm) air. Fracture behavior of the scale was investigated using acoustic emission (AE) analysis during cooling. The water vapor content of the atmosphere has a major influence on the oxidation of the Fe-Cr alloys, whereas it has virtually no effect on the oxidation of Fe. During cooling, significant AE activity starts at about 450°C on the Fe and Fe-Cr alloys oxidized in wet air. This critical temperature is independent of sample composition, oxidation temperature and scale thickness. In the case of the oxidation of the Fe-Cr alloys in dry air, however, the critical temperature shifts to higher temperatures with increasing scale thickness and Cr content. Fracture behavior of the scale should be related to scale structure caused by sample composition and oxidation condition.