Abstract: Generated from coal combustion, Hg can be found in the products from WFGD system, in particular WFGD sludge. The mineral composition and Hg content were studied by X-Ray diffraction(XRD) and scanning electron microscope energy dispersive X-ray(SEM-EDX) in the present study. The partitioning of Hg species in WFGD sludge with different diameter was investigated by Hg temperature programmed decomposition method(Hg-TPD) in the present study. The results indicate that the Hg content in WFGD sludge is 5.652 μg/g which is much higher than that in gypsum. Hg species including HgS,HgO,and HgSO_4 are identified in the WFGD sludge whose proportions are 47%,49%,and 5%,respectively. The proportions of HgO and HgSO_4 decreases when the particular diameter decreased, while the proportion of HgS increased. The percentage of HgS reached over 80% in the sample of 38 μm. Also, the environmental potential risk is evaluated by applying the standard toxicity characteristic leaching procedure. The results indicate that the particular diameter and the Hg species constitution are the dominant factors that influence Hg leaching ability. The sample 74 μm exhibites the highest environmental potential risk with Hg leaching concentration 3.8 μg/g.