Abstract: This paper studies the sulfur composition of sediments and buried peat from Haicang of Fujian province and Futian of Hainan province. The results show that the contents of sulfur in iron-sulfide are much greater than those of the other forms of sulfur. At both sites the contents of organic sulfur are lower, and the contents of the sulfate sulfur are the lowest. The mangrove peat of the Futian site is formed in the upshore intertidal flat and peat flat, and its average sulfur contents are up to 2.60%,and the contents of organic sulfur are well correlated to the organic carbon. However, the correlation between iron-sulfide and organic carbon contents is not very clear. On the other hand, the Haicang site is mainly intertidal flat and tidal channel, with average content of sulfur only 0.43%, and the correlation features between sulfur and organic matter of the sediments are reverse compared with the Futian site. It is noteworthy that the major composition of iron sulphide in coastal mangrove sediments is pyrite and mostly in the form of pyrite framboids. Recent researches have demonstrated that the activity of microorganisms is ubiquitous in mangrove sediments which plays a key role in the formation of pyrite framboids. Therefore, the differences of sulfur contents between the tested coastal mangrove sediments could be explained by the effects of organic matter and sedimentary environment rather than the sulfur source. The characteristics of sulfur in the sediments of coastal mangrove will provide theoretical basis for understanding the formation mechanism of sulfur in coal.