We have invetigated the characteristics of thermal instability of Co-silicides annealed at 850$^{circ}$C ~ 1000$^{circ}$C for 10~90 minutes in a furance with N$_{2}$ ambient. In our experiments, Co-silicides and Co/Ti bilayer silicides were formed by depositing (Co, Ti) films on the clean Si substrates in an E-beam evaporator and performing the RTA annelaing. The sheet resistances of Co-silicides formed form Co exhibited the nearly constant value under the post-annealing time above 900$^{circ}$C showing the increase of 30% and 60% under the conditions annealed at 900$^{circ}$C and 1000$^{circ}$C for 30minutes. On the other hand, there were no remarkable changes in the sheet resistance sof Co-silicides formed form Co/Ti bilayer under the post-annealing conditons below 1000$^{circ}$C.