The electrocatalytic activity of amorphous and crystalline RuO2 thin films for oxygen evolution in an aqueous solution was investigated. The RuO2 films were prepared on FTO substrates by electrodeposition or RF magnetron sputtering technique. The obtained films were annealed at various temperatures. In both cases, the as-prepared films or the 200 C annealed film had an amorphous structure, whereas the films annealed at 300 degrees C and over were crystallized to rutile structure. The analysis of the Tafel slope indicated that the rate-determining step in the oxygen evolution reaction on the amorphous RuO2 was the combination of the adjacent Ru-OH groups, whereas that on crystalline RuO2 was the dissociation of O-H bond in Ru-OH group. The onset potentials of the amorphous RuO2 films for oxygen evolution were shifted toward the negative side by 0.06-0.03 V from those for the rutile crystalline samples. The shift of the onset potential is probably attributed to the structural flexibility which is characteristic of the amorphous surface. This result suggested that the electrocatalytic activity of amorphous RuO2 for oxygen evolution was higher than that of rutile crystalline RuO2. (C) 2010 Elsevier Ltd. All rights reserved.