Abstract: The radiative properties of two different materials surfaces with one-dimensional microscale Gaussian roughness were numerically investigated with an FDTD (finite-difference time-domain) method and near-to-far-field transformation. Calculated results were compared with those from other two approaches. The bi-directional reflectivity peak decreased gradually and eventually disappeared with increasing perfectly electric conductor (PEC) surface RMS (root mean square) roughness. The retroreflection became evident as the ratio of RMS roughness to surface correlation distance increased. Given identical surface characteristics, the overall trend of the bi-directional reflectivity was the same for both PEC and silicon surfaces although the latter was much lower than the former. The reflection of the PEC surface became more like a mirror surface as incident angle increased.