Molten carbonate fuel cells (MCFCs) have shown the highest voltage efficiency among fuel cells. They could be operated over 40000 h under ambient pressure. However, in order to obtain higher performance of MCFCs, especially for high-pressure operation, the stabilities of NiO cathode has been studied experimentally. To obtain lower solubility of Ni2+ ion in the Li-Na eutectic carbonate melt, the effect of additives has been studied. Metal oxides were added to the molten carbonate as well as to solid NiO. The addition of Fe oxide and Mg oxide to NiO was effective in reducing the solubility of NiO. The addition of rare earth oxide (La2O3) to molten carbonate is much more effective in reducing the solubility of NiO. Considering the solubility, MCFCs could operate over 40000 h without a Ni short even under a high-pressure condition of 12-15 atm.