A method of processing a sensing element of an oxygen sensor including an oxygen-ion conductive solid electrolyte, an electrochemical oxygen pumping cell having a first and a second electrode disposed on the solid electrolyte, an electrochemical oxygen sensing cell having a third and a fourth electrode disposed on the solid electrolyte, and diffusion-resistance means for introducing an external measurement gas for contact with the second and third electrodes. An alternating current having a frequency of not higher than 10 Hz is applied to at least one of the oxygen pumping and sensing cells, with the sensing element held at 600.degree. C. or higher, to form minute cracks on the surface of each metal grain of the first and second electrodes and/or third and fourth electrodes. The alternating current is 1-5 times as a large as a polarographic limit current value of the corresponding pumping and sensing cells.