The interface stability of Ta-Mo alloy film on SiO$_2$ was investigated. Ta-Mo alloy films were formed by co-sputtering method, and the alloy composition was varied by controlling Ta and Mo sputtering power, When the atomic composition of Ta was about 91％, the measured work function was 4.24 eV that is suitable for NMOS gate. To identify interface stability between Ta-Mo alloy film and SiO$_2$, C-V and XRD measurements were performed on the samples annealed with rapid thermal processor between $600^{circ}C$ and 90$0^{circ}C$. Even after 90$0^{circ}C$ rapid thermal annealing, excellent interface stability and electrical properties were observed. Also, thermodynamic analysis was studied to compare with experimental results.