We first report that current oscillations with in-situ mass oscillations in the electrocatalytic oxidation of formaldehyde were observed on a Pt electrode. In the linear sweep voltammogram, quantitative analysis of in-situ mass change on the Pt surface by electrochemical quartz crystal microbalance gave a clue that CO migllt be the reaction intermediate species, which covered the active area of Pt surface. It results in the lowest current and the highest mass in a period of oscillations. On the other hand, the lowest value of mass arises from the recover of Pt vacant site due to the CO₂evolution. The periodicity of current oscillations is in excellent agreement with the mass oscillations.