In this paper, automatic reconstruction of building from DSM is presented. Focusing of ROI (regions of interest) is a pivotal step in reconstruction. Contour tracing is adopted here to get the ROI and every layer of building's closed boundary is acquired simultaneously. The normal vector analysis of ROI is used to select the geometric model and reconstruct the roof. The closed boundaries are adjusted by least square refining to constitute the buildings of multi right angles and multi layers. The geometric models used are prismatic (flat roof) and parametric (gabled roof). The experiment shows that the result of reconstruction is satisfactory.