High quality nanometric crystalline Si powders were synthesized from high pure SiH4 by high powerful CO2 laser induced gas phase reaction. The powders produCgd have spherical particle shape with uniform particle size, mean particle diameter of 10～120 nm, and the ratio of mean grain and particle diameter between 0.3～0.7. The sizes of the powders increased with the increase of laser power, reaction pressure, but decreased with the increase of silane gas flow rate and the addition of Ar diluent. Grain sizes dropped distinctly with the rise of the addition of Ar gas and laser power, but changed little with the gas flow rate and reaction temperature. The formation of Si particle undergoes the growth of single crystalline particles and the growth by the collision between these particles.