X-ray diffaction analysis(XRDA)data of the black or brown patina on the surfaces of eight kinds of bronze mirror samples show 4 or 5 broad diffuse peaks which corresponding to almost the same 2θ angle, while their matixes does not.These results indicate that those diffuse peaks were produced by the SnO₂, microcrystals of 3~5 nm size contained in the black or brown patina.The matrix of unearthed black patina bronze mirror numbered 223, gives strong XRDA diffraction peaks, indicating that there are a large amounts of non-oxidized bronze alloy particles surrounded by SnO₂,which shows that the black patinas are not wholly metallic oxides (mainly SnO₂),there are also a lot of original alloys of different sizes in grain in the patina.The bright-field image of the black patina sample gives the same result.