A bronze mirror dating of Han-dynasty was studied in this paper. The in-depth structure and the composition of the natural patina and substrate alloy were determined by Electron probe micro-analyzer (EPMA) observation and EDS cartographies on a cross-section, the mineral composition of patinas were analyzed using XRD. The results show the layer structure of the patina has two obvious layers, and the inner layer ranges from 90µm to 260µm. This bronze mirror is a typical ancient Chinese black mirror Heiqigu, and the formation of high tin layer is due to special surface treatment by the ancients. Tthe composition of the patinas on the mirror and that of substrate alloy were found to be highly heterogeneous, the thin black patina on the mirror containing is cassiterite.