Steam reforming catalyst of methane for DIR-MCFC is easily deactivated by lithium carbonate. In this study, we tried to identify the reason of deactivation why Ni/MgO catalyst for the above reaction loses its activity. The precursor of Ni catalyst supported on MgO was prepared by incipient wetness. The deactivation was attributed to the reaction between catalyst and lithium carbonate resulting in reduction of nickel. Ni/MgO which reacted with lithium carbonate at high temperatures showed both decrease of active site due to sintering and loss of nickel species by formation of solid solution. Therefore, it is concluded that lithium carbonate was capable of causing the sintering and of accelerating the formation of solid solution.