Human exposure to monomethylmercury (MMHg) through seafood consumption is a global concern. This study investigates the potential sources and processes of MMHg in seafood of coastal and freshwater areas through combing of delta 13C, delta 15N, and specific Hg (including MMHg and inorganic Hg (IHg)) isotopes. The results showed that delta 13C and delta 15N values exhibit different patterns in coastal and freshwater species. Delta 199HgMMHg/delta 202HgMMHg values suggested that coastal and freshwater seafood undergo similar aqueous MMHg photodegradation pro-cesses. The Delta 199HgMMHg values could distinguish that, coastal fish absorb MMHg from water column whereas coastal shellfish absorb MMHg mainly from sediment. The positive values of Delta 199HgIHg in seafood could reflect in vivo MMHg demethylation and IHg reabsorption. Positive correlation between delta 15N and Delta 199HgIHg indicated that aquatic organisms in various trophic levels may have different MMHg demethylation efficiency. We proposed that combining of multiple isotopes can provide overall profiles on aquatic MMHg biogeochemical cycle and bioaccumulation.