Electrocatalytic water oxidation was studied using RuCl3 dispersed in a Nafion((R)) membrane coated on a Pt electrode. The coated electrode was at first cathodically polarized at -1.0 V versus Ag \ AgCl to reduce the Ru3 + to Ru-0. The amount of the evolved dioxygen increased with the increase of the amount of Ru moieties in low loaded regions, but decreased in highly loaded regions. A model system containing small catalyst particles in a matrix was studied with a cubic lattice sites model, and the change of the effective surface area against their amount was estimated with a Monte Carlo simulation by excluding inactive surface areas surrounded by particles. According to the simulation, the effective surface area increased with the increase of the amount of the catalyst particles in low loaded regions followed by a decrease in highly loaded regions, which interpreted the activity of the present electrocatalytic water oxidation catalyst.