In this study, we thoroughly investigated the reactive ion etching mechanism of RuO2 film in oxygen plasma with the addition of CF4, Cl-2, and N-2. The etch rate of RuO2 was examined as functions of flow rates of input gases, substrate temperature, DC bias applied to the substrate, and pressure. The concentrations of the etching species in the plasma were determined using optical emission spectroscopy (OES) and a quadrupole mass spectrometer (QMS). The etch products were determined with a QMS and the etched surface of RuO2 film was examined with X-ray photoelectron spectroscopy (XPS). RuO4 and RuO3, which are formed by the reactions between RuO2 film and oxygen radicals, are the only etch products regardless of the kind of additive gas. The additive gases (CF4, Cl-2, and N-2) are not directly involved in the chemical reaction with the RuO2 film, but they increase the concentration of oxygen radicals and accordingly, appreciably increase the etch rate of the RuO2 films. The etch rate is limited by the formation rate of the etch products, which is enhanced by the bombardment of energetic ions. Therefore, the etch rate depends not only on the concentration of oxygen radicals, but also on the nux and energy of the ions bombarding the film surface. In this study, for the first time, we introduced the use of the O-2/N-2 plasma system in RuO2 etching.