As a result of in situ measurements of corrosion behaviors of stainless steel specimens exposed to H2O2 and O2 in high temperature water, the relationship among electrochemical corrosion potential (ECP), metal surface conditions and environmental conditions have been revealed. Based on the relationship, a model to evaluate ECP of stainless steel has been developed with coupling an electrochemical model and a double oxide layer model and then differences in ECP behaviors of specimens exposed to H 2O2 and O2 have been evaluated. Major conclusions are as follows. 1. ECP was determined by the inner oxide layers, which were much affected by OH radicals at the surface. The difference in ECP behaviors of the specimens exposed to H2O2 and O 2 were mainly from the thickness and developing rate of the inner oxide layers. 2. Calculated ECP behaviors, e.g., the different responses to H2O2 and O2 and hysteresis and memory effects, agreed with the measured ones. 3. Further studies were required for fine structures of oxide layers and chemical and physical properties of oxides to improve the accuracy of the model.