Transition metal oxide (TMO) has been extensively focused in recent years. In this paper, we investigate the optical properties of a typical TMO material of Ni(1-x)Mn(2+x)O4 (x=0-1) thin films. Different compositions of x=0, 0.1, 0.2, 0.3 thin films are grown on Pt/Ti/SiO2/Si substrates by chemical solution deposition method under annealing temperature of 750°C. X-ray diffraction patterns indicate that Ni(1-x)Mn(2+x)O4 thin films are polycrystalline with spinel structure. The optical properties are investigated using spectroscopic ellipsometry at room temperature in the wavelength range of 400-1700nm. By fitting the measured ellipsometric data with a three-phase model (air/sample/Pt), the optical constants of thin films are determined. The refractive index and extinction coefficient don't show apparent variation with different composition. The obtained optical constants are very significant in the potential applications of optoelectronic devices.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.