Ni and Ti films were deposited by the thermal evaporator, and then annealed in the N$_2$ ambient at 300-80$0^{circ}C$ in a RTA(rapid thermal annealing) system. Four point probe, AEM, FESEM, AES, and XPS were used to study the effects of Ti-capping layers on the thermal stability of NiSi thin films. The Ti-capped NiSi was stable up to $700^{circ}C$ for 100 sec. RTA, while the uncapped NiSi layers showed high sheet resistance after $600^{circ}C$. These results were due to that the Ni in-diffusion and Si out-diffusion were retarded by the capping layer, resulting in the suppression of the formation of NiSi$_2$and Si grains at the surface.