Fabricating nano-Si by LICVD, we discover: there is a low threshold of laser intensity and a high threshold of SiH 4 flow speed, a nd they are positively correlated to maintain the high temperature for SiH 4 to split. In order to obtain small and even nano-Si particles, the laser intensit y must be large and the SiH 4 flow speed must be fast correspondingly to increa se the nucleation rate of nano-Si particles, and reduce the numbers of Si atoms absorbed by each nano-Si nucleus, and shorten the growth period of ea ch nano-Si nucleus. When nano-Si is annealed for dehydrogeniza tion after it is prepared, its IR absorption spectrum is changed. The positionintensities and shapes of four characteristic absorption spectrum bands are changed respectively,becaus enanoSisurface is large and oxidized so that its constituent is changed.In order to weaken such oxidation,nanoSi should be annealed in Ar but not in the air,and the annealing temperature should be lower than300℃.