Elemental mercury (Hg) emitted from coal-fired power plants and municipal solid waste (MSW) incinerators has caused great harm to the environment and human beings. The strong oxidized OH radicals produced by UV/HO advanced oxidation processes were studied to investigate the performance of Hg removal from simulated flue gases. The results showed that when HO concentration was 1.0 mol/L and the solution pH value was 4.1, the UV/HO system had the highest Hg removal efficiency. The optimal reaction temperature was approximately 50 °C and Hg removal was inhibited when the temperature was higher or lower. The yield of OH radicals during UV/HO reaction was studied by electron paramagnetic resonance (EPR) analysis. UV radiation was the determining factor to remove Hg in UV/HO system due to OH generation during HO decomposition. SO had little influence on Hg removal whereas NO had an inhibitory effect on Hg removal. The detailed findings for Hg removal reactions over UV/HO make it an attractive method for mercury control from flue gases.