The foxing (yellow brown) spots always appear on Chinese paintings made of Xuan zhi (a special Chinese traditional handmade paper). In this study the foxing spots were investigated using analytical techniques which include ultraviolet light, thin-layer chromatography (TCL), liquid chromatography (LC), atomic emission spectroscopy (AES), scanning electron microscope – energy-dispersive X-ray (SEM-EDX), (SEM) photo. The examinations of AES and SEM-EDX show that the iron and copper concentration in the foxed areas is not significantly higher than the unfoxed areas of the same paper. No evidence can be proved the foxing on the Chinese paintings caused by inorganic factors. The analysis of organic acids and amino acids showed that the kinds of acids and their amounts in foxed and unfoxed areas paper were so different. A large amount of malic acid can be found in the extracts of the foxed areas. This may be related with the metabolites of the foxing – causing fungi. Twenty amino acids were detected by liquid chromatography. These amino acid could be specified as aspartic acid, glutamic acid, aminobutyric acid ornithine, serine. In particular, the concentration of aminobutyric acid in the sample S2 and S3 is more than 1000n mol/mL. In all of the unfoxed samples, the concentrations of twenty amino acids are enormously lower than the foxed samples, and even some of the amino acids can not be detected. The amino acids appeared unusually in the foxed areas that could be explained by the fungal body of foxing – causing fungi. Moreover, a great number of growing conidia and mycelia were observed in the incubated samples of foxed area paper. The yellow conidia and brown mycelia cultured fox spots that seems to be related in color with foxing. When these fungi were reinoculated on culture medium, the water drop of yellow-brown secreted by them can been seen. This phenomenon may be used to explain the foxing discoloration migrated, and why hyphae or spore sometimes were not observed in all foxed areas. Probably it is also another evidence that the foxing is formed by fungi. The SEM analysis was undertaken to characterize, identify the causative fungal species. Three kinds of conidia were been found. They are different in the type of ornamentation, size and shape. The investigation of a series of biological proved that the foxing on the xuan zhi is due to the growth of fungi. More than three species of the three species were the foxing causative fungi. The identification of these species have not been completed until now. It is important to study the control of foxing. In addition, the re-infested paper with these fungi try to duplicate the foxing but without success and the climate environment of foxing forming are not well known. The process of foxing forming was formerly considered that need a long time, but recently it has been found that many fox spots appeared on some Chinese paintings at a gallery during two weeks. So the duplication of the fox spots is still worthy to search. The foxing phenomenon is a strong oxidation of the cellulose chain. The pH of foxed areas are more acid than the unfoxed areas of paper. The paper affected by foxing is weaker and friable than the unaffected one. It can be considered as a cause of widespread damage of Chinese painting.