We report on the optical properties and local bonding configurations of both as-deposited and postannealed hydrogenated amorphous silicon nitride (a-SiNx:H) thin films grown on crystalline Si substrates with x approximately 1.2±0.1. Ultraviolet optical reflection and infrared (IR) absorption measurements were applied to characterize the films. A method simply based on optical reflection spectra is proposed for accurate determination of the optical band gap, band tail, wavelength-dependent refractive index and extinction coefficient, as well as the film thickness, suggesting that the Tauc-Lorentz [G. E. Jellison, Jr. and F. A. Modine, Appl. Phys. Lett. 69, 371 (1996); 69, 2137 (1996)] model with the inclusion of Urbach tail is the optimal one to describe the optical response of a-SiNx:H films. The yielded optical parameters can be related well to the film microstructure as revealed by the IR absorption analysis. These results have implications for future deposition controlling and device applications.