Three dimensional (3D) building model is one of the most important components in the construction and representation of urban scenes. For interactive visualization and real-time rendering in cyber city applications, Level of Detail (LOD) technique is useful to reduce unnecessary computations for enhancing visualization system performance. This paper proposes a semi-automatic generalization of 3D polyhedral building models to formulate LOD representations. The idea is to apply generalization in two dimensional (2D) projections of 3D building models and then reconstruct simplified 3D façades accordingly with Boolean-based algorithm. Building roof structures are also simplified with similar techniques and placed on top of the reconstructed façades to complete the generalization of 3D building models. Experimental models with U-, Z-, L-structure and gable roofs were tested to validate the proposed algorithms and to evaluate their performance.