The CoSi 2 layers have been in-situ grown on undoped poly-Si by the reactive chemical vapor deposition of Co(\Eta 5 ?C 5 H 5 )(CO) 2 at 650 ? C and their thermal stabilities have been investigated in the temperature range of 800 to 1000 ? C . The CoSi 2 layer grown by the in-situ method had grains with large area of (111) plane, while grains with little area of (111) plane appeared on the CoSi 2 layer grown by the conventional two-step method where CoSi 2 formed first and transformed to CoSi 2 . The thermal stability of the CoSi 2 layer grown by the in- situ process was improved by more than 100 ? C higher than that of the CoSi 2 layer grown by the conventional two-step process. The CoSi 2 layer grown in situ on a large-grained Poly-Si was stable up to 950 ? C . The effect of stability improvement by the in situ growth was more pronounced when the grain sizes of the poly-Si substrate were small. The improved thermal stability of the in-situ grown CoSi 2 layer could be mainly due to the formation of a uniform CoSi 2 layer with the CoSi 2 grains, which are in the form of epitaxial-like growth on the each poly-Si grains, causing a reduction of the interfacial energy of the system.