Thin NiO films were deposited at 500 degrees C on n-type Si(1 1 1) by a DC reactive magnetron sputtering in a gas mixture of oxygen and argon. The ratio between the flow rates of oxygen and argon was respectively set at 1:4, 1:2, and 1:1. The dependence of structures and optical properties of NiO films were investigated using grazing incidence X-ray diffraction and spectroscopic ellipsometry in the spectral region of 1.5-5.0 eV. Ni-rich NiO films were obtained when the ratio between the flow rates of oxygen and argon was 1:4 and 1: 2 in sputtering process. And when the ratio was 1:1, a relatively pure NiO film was formed. The partial pressure of oxygen could significantly influence the thickness and roughness of films. Refractive index n, extinction coefficient k, and direct gap energy and indirect gap energy of the NiO films were also subject to the influence of the partial pressure of oxygen. (C) 2011 Elsevier B.V. All rights reserved.