Uniform polycrystalline $CoSi_2$layers have been grown in situ on a polycrystalline Si substrate at temperature near $625^{circ}C$ 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 $600^{circ}C$ to $900^{circ}C$. The $CoSi_2$layer was degraded at $900^{circ}C$. Inserting a TiN interlayer between polycrystalline Si and $_CoSi2$layers improved the thermal stability of $CoSi_2$layer up to $900^{circ}C$ due to the suppression of the Co diffusion.