The measurement of the optical constants for a rough surface thin film is needed for CVD polycrystalline diamond films which typically have surface roughnesses of the order of tens to hundreds of nm. In this paper, we present a method for determining the optical constants n and k as well as the surface roughness σ for a rough surface diamond thin film. By using the theory of optical reflectance (R) and transmittance (T) for a thin film with incorporation of rough surface scattering factors which are functions of the surface roughness a, the following function can be obtained: R=R(n,k,σ,λ,d), T=T(n,k,σ,λ,d), and Rp=Rp(n,k,σ,λ,d), where λ and d are the incident wavelength and film thickness, respectively. Rp is the reflectance from the smooth back surface of the thin film. From the experimental data for R, T, and Rp in a range of wavelengths, the optical constants n, k, and σ can be extracted by solving the above non-linear equations using a three-dimensional Newton-Raphson technique. Examples of experimental results on diamond films and other examples of simulated results are presented to illustrate the usefulness and validity of this technique.