We have investigated formation of CoSi2 on various grain sizes of polycrystalline Si (poly-Si) with emphasis on iis thermal stability. As the grain size of poly-Si decreases, CoSi2 phase is formed at lower temperature because of the diffusion of Co atoms along grain boundaries of poly-Si during the rapid thermal annealing process. The enhanced reaction of cobalt with silicon on small-grain-sized poly-Si creates a rough CoSi2/poly-Si interface, which becomes thermally unstable. CoSi2 formed on amorphous Si showed less thermal stability than that found on medium and large grain sized poly-Si.