We have developed a solid oxide fuel cell (SOFC) stack with an internal manifold structure. The stack, which is composed of 25 anode-supported 100-mm-diameter SOFCs, provided an electrical conversion efficiency of 56% (based an the lower heating value of methane, which was used as a fuel) and an output of 350 W when the fuel utilization, current density, and operating temperature were 75%, 0.3 A cm(-2), and 1073 K, respectively. The electrical efficiency and the output were maintained for 1100 h. The cell voltage fluctuation was 2% for 25 cells. The relationship between average cell voltage and current density in the 25-cell stack was as almost the same as that in the 1- and 10-cell stacks, which suggests that our stack provides almost the same cell performance regardless the number of the cells. (C) 2007 Elsevier B.V. All rights reserved.