Nanocrystalline Si clusters can be formed in thermally annealed a-Si: H films. Using different characterizing techniques such as micro-Raman scattering,and X-ray diffraction,we have found nc-Si clusters distribute uniformly in the amorphous matrix of the annealed films with typical sizes in the range of 1.6～15 nm in diameter. The sizes of these silicon particles change sensitively with the temperature ramp rate applied during thermal annealing.