In vacuum environment, the nano-crystalline silicon films were prepared by pulsed laser ablation at high temperature and room temperature respectively. The amorphous films prepared under normal temperature were thermal-annealed, which leads to crystallization. The morphology and compositon etc. of the samples were characterized by scanning electron microscopy, Raman scattering and X-ray diffraction. The results showed that the temperature threshold of Si nanoparticles formation was 700 ℃ and 850 ℃ respectively. The nucleation energy of the nanoparticles was obtained by quantitative calculation, and the reason of difference between the temperature threshold was discussed from the point of view of energy.