Concentrations and distributions of various mercury (Hg) species, including total Hg (HgT), dissolved Hg (HgD), reactive Hg (HgR), particulate Hg (HgP), total methylmercury (MeHgT), dissolved methylmercury (MeHgD), and particulate methylmercury (MeHgP) in surface water and porewater of the sediment collected from Lake Caohai during the flood and dry seasons were analyzed by using CVAFS and GC techniques. The concentrations of HgT ranged from 1.7 to 9.0ng/L, HgR from 0.06 to 1.4ng/L, and MeHgT from 0.11 to 0.67ng/L in surface water, respectively. In the sediment porewater, the concentrations of HgD varied from 8.6 to 39.6ng/L, and MeHgD ranged from 0.11 to 4.9ng/L. The Hg species in surface water was mostly dominated with dissolved forms with an average of 87% of HgT in dry flow period and 51% of HgT in flood flow period, respectively. The HgD and HgP forms were significantly correlated with HgT. The MeHgD were positively correlated to MeHgT, showing that distributions of MeHgT were controlled by MeHgD. Concentrations of HgD and MeHgD in porewater were higher than that obtained in the underlying water, which suggestted that the sediment was a dominant Hg source to the lake.