[ $SiO_{2}$ ] or SiON is usually deposited and annealed after formation of silicide in real transistor fabrication processes. Nickel silicide and nickel silicide with Co interlayer were annealed at 650$^{circ}C$ for 30 min with silica top layer in this study to investigate its thermal stability. SEM, XPS, and FPP(four point probe) were employed for the investigation. Nickel silicide with Co interlayer showed improved thermal stability. Co interlayer seems to play a key role to the stability of nickel silicide.