In general, black patina is formed upon the surface of the ancient bronze mirrors. Recently, a kind of black patina is found on an ancient bronze sword, which was made 2500 years ago during the Warring States Period of China. In this paper, chemical compositions and microstructures of the black patina were characterized by using instruments including optical microscope (OM), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), X-ray diffraction (XRD) and Raman spectrometer (Raman). The results revealed that the formation of the black patina on the bronze sword is due to two reasons: (1) high fin (Sn) content reaches to 22.92 wt%; (2) damp, wet and acid burial environment. Therefore, it conforms to the natural corrosion formation mechanism, i.e., after selective dissolution of elements Cu and Pb from the matrix, element Sn was oxidized and then, the gel-like SnO2 center dot xH(2)O filled the vacancies left by the selective corrosion of Cu and Pb. This work will be helpful for better restoration and preservation of the bronze cultural relics.