This paper presents a new way to study passivation mechanism of SiN x -Si interface using capacitance-voltage method. Fixed charge density (N f ) near dielectric/Si interface, which is closely related to field effect passivation, and interface trap density (D it ) at dielectric/Si interface, which is closely related to chemical passivation, can be obtained directly from experimental CV characteristics. The passivation properties of SiN x -Si can be studied and optimized by the MIS model.