In this paper, we give the principle and the empirical formulas of the preparation for nanometer sized a-Si3N4 particles by laser induced chemical vapor deposition (LICVD). Under certain technological parameters, our experiments have obtained high quality nanometer a-Si3N4 particles whose average diameter is 6.5nm. We disperse these particles in organic solvent so as to study its energy level structure with ultravolet-visible spectrum. The photoluminescent experiments show its peak-like spectrum structure and the splitting of energy band, which relate to super micro-particles. We also describe the picture of the physical structure of nanometer a-Si3N4 particles and demonstrate that silicon"wrong bond"≡Si-Si≡and silicon dangling bond≡Si30 play a dominant role in the spectrum character for Si-riched nanometer a-Si3N4 particle.