Optimum control of mercury released from the coal-fired power plant is evaluated by determining its efficiency and appropriateness in reducing emissions and ambient air concentrations. The 2400 MW power plant fueled by lignite located in Thailand is demonstrated in this study. Emissions of mercury from the coal-fired power plant are calculated under 3 major scenarios. The first scenario is the amount of mercury released under the existing operation of the power plant. Emission rate of mercury is calculated as 41 g/h which indicates a co-benefit of mercury removal from the installation of existing conventional air pollution treatment systems (electrostatic precipitator and wet flue gas desulfurization) as compare with the 2 nd scenario of without equipping of air pollution control devices at the power plant (374 g/h of mercury emission). Adding controlling measures to existing operation of the power plant can lead to decreasing of mercury emissions at different levels. The relationship between changing of emissions affected to ambient air concentrations of mercury is evaluated using the CALPUFF air dispersion model. Results indicate small decreasing of predicted ambient concentrations after applying additional mercury control measures to the BAU of the power plant. This study reveals the co-benefit of existing air pollution treatment devices in controlling mercury emission. It also illustrates that the efficiency and appropriateness of current air pollution control system is in an optimal and acceptable levels in mercury control.Finding and methodology in this study can be used as a case study in quantitative evaluation of the effectiveness and appropriateness of environmental control mitigation measures added to the existing operations. It clearly illustrates the need to analyze the co-benefit of current air pollution control system towards the accomplishment on controlling emissions of other emerging air pollutants which will provide the best optimum air pollution control to the emission source.