Thermal stability of RuO2 thin films formed by modified atomic layer deposition on SiO2 substrate was investigated. Rapid thermal annealing was conducted for 2 min under NH3 and N2 ambient. It was demonstrated that NH3 gas can completely reduce RuO2 to pure Ru at a relatively low annealing temperature of 500 °C, while partial reduction of RuO2 into Ru at the outmost surface was observed after N2 annealing. Agglomeration of the NH3 annealed film was not observed due to high quality of the as-deposited film that was dense and had low level of impurities such as hydrogen and carbon.