Abstract: Crystallization process of CoSi2 thin films prepared by radio frequency magnetron sputtering have been investigated by in situ X-ray diffraction. The results show that the as-deposited Co-Si film is amorphous. The gibbs free energy-composition curve proved that amorphous phase has lower Gibbs free energy. The first formed phase during crystallization is CoSi phase, its formation is determined by EHF (effective heat of formation) and structure factors. As the temperature was elevated, the amorphous thin film finally transformed to crystalline CoSi2.