Ti1-xNbxN thin films with x=0, 0.26, 0.41, 0.58 and 1 were deposited on silicon (311) substrate by RF magnetron sputtering. The dielectric functions of these films were calculated by fitting measured reflectance spectra to the Drude-Lorentz model. The measured reflectance spectra exhibits a minimum in the visible region and this feature shifts to higher energy (shorter wavelength) with increase in x. The observed behavior can be modeled as the response of four Lorentz oscillators. The real part of the dielectric function is characterized by a screened plasma energy of 2.26 eV for x=0 which increased to 2.80 eV for x=0.58 in the Ti1-xNbxN film. © 2012 American Institute of Physics.