The second-order nonlinearity of Si-rich SiNx thin films prepared by RF sputtering was investigated. It was found that the second-order nonlinear coefficient (deff) increases with increasing Si concentration in a wide range of Si concentrations. The observed maximum value of deff was 5.9 pm V−1, which is twice that of the β-BaB2O4 crystal. X-ray photoelectron spectroscopy analysis in the Si 2p energy region revealed that the films consist of Si species with different valence states. Strong correlation was observed between the deff values and the XPS peak intensity of Si0 species, suggesting that Si clusters are the most probable origin of the large second-order nonlinearity.