304 stainless steel (SS) was exposed to 325 degrees C water condition to simulate the operation condition of nuclear power reactor. The microstructural characteristics and corrosion behaviors of exposed 304 SS was investigated. Result shows that the exposed 304 SS surface form an oxide film, which consist of outer film and inner film. The outer film is composed of mu m-size hematite and nm-size spinel oxide particles formed on top of the original metal surface. The continuous inner film contains Nano-crystalline spinel. Potentiodynamic polarization test results indicate that the corrosion behavior of 304 SS decrease with exposure time increasing. Because the quantity of pores on oxide film increase with exposure time increasing, and the corrosion behavior decrease. The oxide film gradually grows with exposure time. Finally, the formation and evolution mechanism of oxide film was proposed and the determining factors was discussed.