In natural environment, tarnish was observed on the surface of a commemoration silver coin of China. In order to identify the nature of the tarnishing film, optical microscope(OM), scanning electron microscope(SEM), electron microprobe analyses(EMPA), X-ray photoelectron spectroscopy(XPS), and X-ray diffraction(XRD) were applied to examine the tarnishing areas. It was found that the tarnishing position is uniformly moderate brown and several dark brown spots are randomly distributed. Groups of corrosion holes are distributed over the forging stress zones formed in the struck process. As result of surface analyses using EMPA, sulfur and oxygen were detected besides silver with uniformity of sulfur. Furthermore, distributions of silver and oxygen were in correspondence with the morphology of dark brown spots. XPS and XRD showed that components of spots are Ag2S, Ag2O and Ag2SO3, with a predominance of the first one. It could be estimated that the tarnish of silver is a series of electrochemical process that requires the presence of sulfur and oxygen.