In order to elucidate the reaction mechanism at the anode of solid oxide fuel cells (SOFC) in H-2-H2O atmospheres, instead of conventional nickel-zirconia cermet anodes, we employed nickel stripe pattern electrodes prepared on the surface of 8m/o Y2O3-doped ZrO2 (YSZ), which have well-defined length and morphology of the gas/nickel/YSZ triplet phase boundary (TPB). On the surface of single crystalline YSZ-plates, compact nickel layer was prepared by ICB (ionized cluster beam) method. By photolithography, the stripe patterns of alternative nickel and YSZ lines were prepared. Electrochemical measurements were made on the electrode impedance and steady-state polarization current using a three terminal method in the H-2-H2O-Ar gas mixtures with hydrogen partial pressure, P(H-2) = 10(2)-10(4) Pa and water vapor pressure, P(H2O) = 3 X 10(2)-2 X 10(3)Pa at 700-degrees-C. It was shown that the rate of electrode reaction at 700-degrees-C can be approximately expressed by I = kP(H-2)a(o)-k’P(H2O)1/2a(o)-1/2 (k, k’ : rate constant), where a(O) is the oxygen activity at the TPB which is related to the electrode potential E versus 1.01 3 X 10(5)Pa O-2(g) by RT ln a(O) = 2EF. The rate of anodic reaction was found to be essentially determined by the reaction of H-2 (g) and the adsorbed oxygen on the nickel surface, while the rate of cathodic reaction seems to be determined by the first order reaction or surface diffusion process of adsorbed hydrogen, H(ad) which may take place after the reaction of H2O(g) --> H2O(ad) --> H(ad) + OH(ad).