Co-combustion of sludge with coal is proposed as an alternative option for sludge disposal instead of ocean dumping. Because the combustion of sludge can cause emissions of various air pollutants, it is important to understand the characteristics of sludge combustion flue gases. Especially, very few studies are found to investigate mercury emissions from the co-combustion of sludge with coal. In this study, coal and sludge samples were prepared with different mixing ratios. These samples were combusted in a bench-scale combustor, and their flue gases were analyzed. Due to higher mercury content in the sludge sample, higher mercury emissions were found with an increase in a ratio of sludge to coal. Compared to the combustion flue gas of the coal sample, higher levels of mercury oxidation were found in the combustion flue gas of the samples including sludge.