To study effects of nitrate (NO3(-)) on monomethylmercury (MMHg) photodecomposition (PD), laboratory experiments were conducted to investigate the role of NO3(-) in MMHg PD under various light radiations, and to examine effects of NO3(-) concentration gradients on MMHg PD rates and end products. We analysed the react processes according to Hg2+ concentration. The results indicated that in the reactor exposed to natural and ultraviolet radiation, and treated with NO3(-), the rates of MMHg PD were calculated to be 0.10 L x (ng x h)(-1) and 0.046 L x (ng x h)(-1), respectively, Hg(0) fluxes were 1.05 and 1.27 ng, respectively, and Hg2+ concentrations were 16.97 ng x L(-1) and 28.92 ng x L(-1) respectively. In the experiments which were not spiked with nitrate or spiked with benzoic acid, MMHg PD rates were calculated to be 0.052 L x (ng x h)(-1) and 0.015 L x (ng x h)(-1), respectively, Hg(0) fluxes were 23.81 ng and 15.38 ng, respectively, and all concentrations of Hg2+ (< 10 ng x L(-1)) represented a trend of increasing firstly and then decreasing. There were no differences among the reactions exposed to visible light (P = 0.56), the PD rate was about 0.003 L x (ng x h)(-1), Hg(0) flux was about 5 ng, and Hg2+ concentrations showed a trend of increasing firstly and then decreasing. In the reactor exposed to natural light, with the increasing concentration of NO3(-), MMHg PD rate increased, Hg(0) flux decreased, and Hg2+ concentration increased with respect to time. All results indicate that NO3(-) has a significant effect on the react process, PD rate, and end products.