$RuO_2$ thin films were deposited on Si and Ru/Si substrates by rf magnetron reactive sputtering and annealed in oxygen atmosphere(1atm) to investigate their thermal stability and diffusion barrier property. $RuO_2$ thin films were thermally stable up to 700ulcorner for 10min. in oxygen atmosphere and showed excellent barrier property against the interdiffusion of silicon and oxygen. After annealing at $750^{circ}C$ , however, volatilization to higher oxide occurred at the surface and inside of $RuO_2$ thin film and diffusion barrier property was also deteriorated. When annealed at $800^{circ}C$, $RuO_2$thin film showed a different microstructure from that of $RuO_2$ thin film annealed at 75$0^{circ}C$. It is likely that surface defect structure of $RuO_2$, $RuO_3$, and excess oxygen had an influence on the mode of volatilization with increasing annealing temperature.