The oxygen diffusion current on the platinum electrode was studied theoretically in both circular and band type electrodes. For the oxygen current of the circular electrode the following equation was obtained; I=4naFDC₀, where a is the radius of the electrode, D the diffusion coefficient and C₀ the oxygen concentration in the bulk solution. The current of the band electrode was independent of the thickness of the band, as shown by the following; I=nΠlFDC₂/Ψ, where l is the length of the band and Ψ is the parameter showing the extension of the boundary of the diffusion region; outside the boundary the concentration is maintained at a constant C₀. The theoretical equation of the oxygen current of the circular electrode agreed well qualitatively with the experimental result. In the case of the band electrode, the oxygen diffusion current was found to be independent of the thickness; this was approximately ascertained by the experimental data.