In this study, the characteristics of low-pressure chemical vapor deposition deposited silicon nitride films on n-GaN substrate were investigated. The physical and chemical properties were measured and surveyed. And an Al/LPCVD-Si3N4/n-GaN MOS structure was used for the electrical characterizations. For the electrical property improvements, we investigated the low-pressure chemical vapor deposition deposited silicon nitride films by (NH4)2Sx treatment. Furthermore, the silicon nitride films were passivated by fluorine ions to improve the electrical characterizations that came from the liquid phase deposited SiO2 stacks. After the (NH4)2Sx treatment and fluorine ions passivation, the dielectric constant of low-pressure chemical vapor deposition deposited silicon nitride films were maintained and the leakage current density were improved. The highest dielectric constant is 12.13, and lowest leakage current density are 1.73¡Ñ10-10 A/cm2 at 1 MV/cm and 3.81¡Ñ10-10 A/cm2 at 1 MV/cm for the LPCVD-Si3N4 film after fluorine passivation and (NH4)2Sx treatment.