Uniform polycrystalline CoSi 2 layers have been grown in situ on a polycrystalline Si substrate at temperature near 625 ? C by reactive chemical vapor deposition of cyclopentadienyl dicarbonyl cobalt, Co(η 5 -C 5 H 5 )(CO) 2 . The growth behavior and thermal stability of CoSi 2 layer grown on polycrystalline Si substrates were investigated. The plate-like CoSi 2 was initially formed with either (111), (220) or (311) interface on polycrystalline Si substrate. As deposition time was increasing, a uniform epitaxial CoSi 2 layer was grown from the discrete CoSi 2 plate, where the orientation of the CoSi 2 layer is same as the orientation of polycrystalline Si grain. The interface between CoSi 2 layer and polycrystalline Si substrate was always (111) coherent. The growth of the uniform CoSi 2 layer had a parabolic relationship with the deposition time. Therefore we confirmed that the growth of CoSi 2 layer was controlled by diffusion of cobalt. The thermal stability of CoSi 2 layer on small grain-sized polycrystalline Si substrate has been investigated using sheet resistance measurement at temperature from 600 ? C to 900 ? C . The CoSi 2 layer was degraded at 900 ? C . Inserting a TiN interlayer between polycrystalline Si and C oSi2 layers improved the thermal stability of CoSi 2 layer up to 900 ? C due to the suppression of the Co diffusion.