The silicon nitride films are prepared on c-Si substrates by plasma enhanced chemical deposition (PECVD) with silane as the silicon source in mixture gas (N2/NH3) as the nitrogen source. We prepare different kinds of films at different flow rates of the nitrogen with other conditions kept the same. X-ray diffraction (XRD) is employed to analyze the crystal structure, and the existence of the silicon nanoparticles embedded in the silicon nitride film is verified according to the caculation of the lattice size. Fourier transform infrared spectra are employed to probe the concentration evolutions of various chemical bonds with the flow rate of the nitrogen, with which by combining the chemical reaction process, the formation mechanism of the silicon nano-clusters embedded in silicon nitride is investigated. The results show the initial positions of silicon nanoparticles are conducible to the formation of silicon nanoparticles when the chemical reaction proceeds towards the direction in which the SiSi bonds form. In addition, XRD analysis and photoluminescence characteristics show that the size and the concentration of the embedded nanoparticles increase with the flow rate of the nitrogen increasing.