Activated carbon is a widely used sorbent for the removal of mercury from coal combustion flue gas. Many studies have been conducted to understand the physical properties of activated carbon and the flue gas conditions that are important for increasing its mercury adsorption capability. The Brunauer-Emmett-Teller (BET) surface area and pore volume of activated carbon and the injection of acidic gases have been reported to influence the performance of activated carbon in mercury adsorption. However, the mercury adsorption mechanism of activated carbon is not understood in terms of the physical properties of activated carbon and the composition of the flue gas. Therefore, two representative raw materials of activated carbon, wood and coal, were used in this study. Activated carbon samples with varying physical properties were prepared by applying different carbonization and activation conditions. The prepared activated carbons were tested for mercury adsorption in three different simulated gas compositions: (1) a baseline gas condition and (2) two simulated flue gas conditions. The mercury adsorption efficiency was shown to linearly increase with the BET surface area of activated carbon in the baseline gas condition. The coal activated carbon showed a higher mercury adsorption efficiency than the pinewood activated carbon. In the simulated flue gas conditions, the mercury adsorption efficiency rapidly increased and then very slightly increased with each physical property of activated carbon.