Zinc nitride (Zn3N2) thin films were deposited on glass substrates by reactive radio-frequency magnetron sputtering from a pure Zn target in nitrogen-argon ambient. X-ray diffraction analysis indicates that the films just after the deposition are polycrystalline with a preferred orientation of (400). With increasing substrate temperature, the grain size in zinc nitride film increases from 26.5 nm (100 ℃) to 33.6 nm (200 ℃), and then decreases to 17.8 nm (300℃). Atomic force microscopy reveals that the film surface morphology is dependent on the substrate temperature. With reflective spectroscopic ellipsometry, the ellipsometric parameters ψ and Δ of Zn3N2 films are measured. Then, a new model for Zn3N2 films is built. With the Tauc-Lorentz dispersion formula, the ellipsometric data are fitted, and both the thickness and optical constants (refractive index and extinction coefficient) of the films are obtained at a wavelength of 430–850 nm. The optical band gap is calculated from the extinction coefficient by using the Tauc formula, and a direct band gap of 1.73–1.79 eV is obtained.