The extraction of building’s roof is an important step in 3D reconstruction of building. In order to more precisely extract the roof information, both high-resolution optical and SAR images were fused in this paper. Layerover of building in SAR image was used to determine that building on optical image, and thereafter a small window was setup including building’s roof. The ISODATA classification method was used to obtain an outline of building’s roof for the small window. Finally, the regular outline of the roof was computed. The experiment proved that this method was feasible.