Mercury (Hg°) removal process for coal derived fuel gas in the IGCC process will be one of the important issues for the development of a clean and highly efficient coal power generation system. Recently iron based sorbents such as iron oxide (Fe203), supported iron oxides on Ti02, and iron sulfides were proposed as active mercury sorbent. It was supposed that Hg° reacted with H2S over the sorbents. The H2S is one of the main impurity compounds in coal derived fuel gas, therefore H2S injection is not necessary in this system. HCI is also another impurity in coal derived fuel gas. In this study, the contribution of HCI to the mercury removal from coal derived fuel gas by a commercial activated carbon (AC) was studied using a temperature programmed decomposition desorption (TPDD) technique. TPDD technique was applied to understand the decomposition character of the mercury species on the sorbents. The Hg° removal experiments were carried out in a laboratory-scale fixed-bed reactor at 80-300 °C using a simulated fuel gas. The following results were obtained from this study: 1) HCI contributed to the mercury removal from the coal derived fuel gas by the AC; 2) H2S suppressed the mercury removal with HCI by the AC; 3) The stability of the mercury surface species formed on the AC in the presence of H2S was different from that in the absence of H2S.