Silicon nitride has better high temperature capabilities than most metals combining retention of high strength and creep resistance with oxidation resistance. In addition, its low thermal expansion coefficient gives good thermal shock resistance compared with most ceramic materials. Silicon nitride is widely used in semiconductor process and micro-electromechanical systems (MEMS) for its advanced material properties. The silicon nitride thin films prepared by low pressure chemical vapor deposition (LPVCD) technique at the deposition temperature of 950℃ shows strong greenish-blue visible photoluminescence (PL) emission with five separate peaks at 2.7, 2.69, 2.4, 2.3 and 2.1 eV, respectively, by the 3.45 eV laser excitation, at room temperature. The results of TEM, IR and X-ray photoelectron spectroscopy(XPS) show that the N/Si ratio of the films is 0.75,which means the film is silicon-rich, and the film is constructed with mosaic crystal silicon nanoclusters embedded in silicon nitride solid, the dimension of which is about tens nanometer. To find out the luminescence property of as-deposited thin films, a series of LPCVD SiN_x films was treated by rapid thermal annealing (RTA) for 15 s in N₂ at 1000,1100 and 1200℃, and we analyzed the changes in PL spectra, composition and microstructure caused by the RTA process. We found after RTA, the relative intensity of some peaks changed, and two new photoluminescence peaks appears at 3.0 eV and 2.8 eV.We believe the peaks at 2.7,2.3,2.1 eV is separately caused by the electronic transition of ≡Si⁰→N—Si—O, ≡Si⁰→N^-, E_c→≡Si^-, the peaks at 2.69 and 2.4 eV is caused by the electronic transition of ≡Si⁰→Si—O—Si, E_c→≡Si⁰.Those peaks will change after RTA, for the thin film’s composition and defect state density can be changed by RTA process. And we also found the nano-silicon microstructure can greatly decreased the stress of SiN_x film (to 10⁷ Pa), which can help us to prepare high quality silicon nitride films.