Technological feasibility of ammonia-fueled solid oxide fuel cell (SOFC) systems are investigated with SOFC stacks consisting of 30 planar anode-supported cells. SOFC systems are investigated with and without external ammonia decomposition. BaO/Ni/Sm2O3/MgO catalyst is newly developed for the external ammonia cracker. Initial performance measurement and 1,000 h long-term durability test of the stacks are conducted. The stack with direct ammonia achieves 1 kW power output with 52% DC electrical efficiency; a slight decrease in performance compared with the stack with mixture fuel of hydrogen and nitrogen is because of the temperature decrease in the stack owing to the endothermic ammonia decomposition reaction. The external ammonia cracker helps to maintain stack temperature, enhancing the initial performance of the stack. It is demonstrated that the stacks fueled with ammonia have excellent stability during the long-term test and that with the external ammonia cracker exhibits 57% energy conversion efficiency at ca. 700 W DC electrical output.