In this paper, we present a method for determining the refractive index, extinction coefficient, and thickness of thin films using reflectance and transmittance measurements for one frequency of incident light.The measurement of reflectance is conducted from both sides of the sample and transmittance from either one of the two sides.All measurements are conducted at the same angle of incidence with either s- or p-polarized light.The usual assumptions about the sample (substrate with deposited thin film) are made: (1) homogeneity of the deposited thin film; (2) uniform film thickness over the measurement area; (3) non-absorbing incoherent substrate; (4) the film is absorbing, i.e.the extinction coefficient is not 0. A computer program to solve the three equations, two equations for reflectance and one equation for transmittance, to determine the refractive index, extinction coefficient, and thickness of thin film has been developed and tested on simulated data.