Abstract: The realgar As4S4 and orpiment As2S3, used as yellow pigment in traditional Chinese painting, are low toxic minerals. However, it is well known that the arsenic As2O3 is a virulent poison. The toxic effect of arsenide is relative to its water-solubility. In order to study the stabilities and water-solubility of realgar and orpiment, the arsenide As4S4, As2S3, As2O3, As2O5, As4S3, and As4S5 were calculated by density functional theory B3LYP/6-31G(d) method. The results reveal that the energies of valent orbitals in S atom are higher than those in O atom and closer to those in As atom. Therefore, the As-S bonds are much stronger than the As-O bonds, and the polarity of As-S bonds is much weaker than the As-O bonds. Besides the strong As-S bonds, there is strong As-As bond in arsenic sulfides. This is the reason of high stabilities of arsenic sulfides. Contrary to the notable positive charge of As and negative charge of O in As2O3 and As2O5, the little positive charge on As and negative charge on S lead to thevery weak interactions between arsenic sulfides and water. Therefore, all the arsenic sulfides possess poor water solubility. Especially for the As4S4 and As2S3 possessing D2d and D3h symmetry respectively, they are all nonpolar molecules and more poorly soluble in water. This results in the low toxicity of As4S4 and As2S3. As a result, the murals or picture posters painted with realgar and orpiment can not be spoiled by dampness and rain.