The basic principles of silicon nitride nano-powder synthesis by laser induced chem-ical vapor deposition method are investigated. By adding an ultraviolet beam of light,which is perpendicular to CO₂ laser,the concentration of dissociated Si atoms decreased because then(N)/n(Si)ratio increased. The powders possess the following characteristics: high purity,ideal stoichiometry(n(N)/n(Si)=1.314)and an narrow range of size(7～15 nm). Transmission electron graph shows that the powders are nearly monodispersed. The“ blue shift” and“widening” in the observed infrared spectra and Raman spectra are resulted from the surface effect and quantum size. The laser induced chemical vapor deposition method with double beam optical stimulation is a preferable way to synthesize silicon nitride nano-powder of high purity.