In borate buffer solution, anodic oxide passive film was formed on 316SS at 0.6 VSCE. Chemical composition and semiconductor properties of the film were studied by XPS and capacitance measurement. It’s determined from the results that the film has duplex structure with inner oxide layer and outer hydroxide layer. The inner layer behaves as p-type semiconductor, while the outer layer shows n-type semiconductor. A p-n junction is formed at the interface of outer and inner layer, and it plays an important role in the protective effect on the metal matrix.