Spectral reflectance of GaAs from infrared (IR) to ultra-violet (UV) bands is predicted using ab initio calculations. We first predict the spectral dielectric function. Two major mechanisms exist for different photon wavelength, namely, photon-electron coupling in the UV to near-IR region and photon-phonon coupling in the far-IR region. For the near-IR to UV band, the electronic band structure of GaAs is calculated, and the imaginary part of the dielectric function is determined from the band structure using the Fermi's golden rule. The real part of spectral dielectric function is then derived from Kramer-Kronig transformation. For the far-IR region, ab initio calculations are used to determine the phonon modes. and the dielectric function is then predicted using the oscillator model. The spectral reflectance for the entire spectrum is then calculated using Fresnel's law for a semi-infinite GaAs slab. The predicted results agree reasonably well with experimental data, demonstrating the capability of ab initio calculations to predict thermal radiative properties of semiconductor materials from their atomic structures. (C) 2009 Elsevier Ltd. All rights reserved.