Arsenic is a common hazardous element in coal. It is always difficultto characterize its modes of occurrence quantitatively because of its low concentration in coal. In recent years, sequential chemical extract experiments were employed to determine modes of occurrence of arsenic quantitatively. The following conclusions can be drawn from these experimental data: ①The modes of occurrence of arsenic in coal include sulfide arsenic, organic arsenic, arsenate arsenic, silicate arsenic, soluble and exchangeable arsenic. Generally, the percentage sequence of arsenic in different states are as follows: sulfide arsenic＞organic arsenic＞arsenate arsenic＞silicate arsenic＞soluble and exchangeable arsenic. However, modes of occurrence of arsenic in different coal samples show big difference. ②Generally speaking, most of arsenic in coal are associated with arsenic-bearing pyrite, and arsenic contents of pyrite are related to origin or genetic type of pyrite. Arsenic of arsenate in coal is mainly associated with Fe-oxides and Fe-hydroxide. Arsenic of silicate mainly comes into crystal lattice of clay minerals. ③Low-arsenic coals often have high organic arsenic. Humic acid and fulvic acid extracted from lignite and low rank bituminous coal also combine some of arsenic. However, chemical structure of organic arsenic in coal is still unclear currently. ④The modes of occurrence of arsenic in super-high arsenic coal from Guizhou province are so complicated, and organic arsenic combined with oxygen are dominant occurrence of arsenic in some of these high-arsenic coals.