Tubular flat-plate (TFP)-solid oxide fuel cells (SOFCs) supported by cathodes with gas flow channels were fabricated by co-firing the cathodes and electrolytes. The size of a single cell is 43 mm x 4.5 nun x 100 mm, and the maximum power density of 0.83 W/cm(2) was achieved at a current density of 1.6 A/cm(2) at 1000 degreesC. We studied the electrochemical characteristics of the anode and the cathode by the ac impedance method. The resistance corresponding to the reaction around the three-phase boundaries of the anode/electrolyte interface increases with decreasing temperature from 1000 to 800 degreesC in the Ni-Y2O3 stabilized ZrO2 (YSZ) anode of the TFP cell. From ac impedance measurements made at various oxygen partial pressures P-O2, we found that under high P-O2 (log P-O2 > -0.77) surface diffusion of adsorbed oxygen atoms was the dominant step, but under low P-O2 (log P-O2 < -0.77) the diffusion of oxygen gas was the rate-determining step in the cathode reaction in this TFP cell.