This paper presents an extension of our previous work on decomposition-based assembly synthesis [1], where the 3D finite element model of a vehicle body-in-white (BIW) is optimally decomposed into a set of components considering the stiffness of the assembled structure under given loading conditions, and the manufacturability and assemblability of each component. The stiffness of the assembled structure is evaluated by finite element analyses, where spot-welded joints are modeled as linear torsional springs. In order to allow close examinations of the trade-off among stiffness, manufacturability, and assemblability, the optimal decomposition problem is solved by multi-objective genetic algorithm [2,3], with graph-based crossover [4,5], combined with FEM analyses, generating Pareto optimal solutions. Two software programs are developed to implement the proposed method. The first software, ASPre, allows designers to interactively decide potentially decomposable components using a FE model imported from commercial CAE software. Using the outputs of the ASPre, the second software, ASOpt, solves the optimal decomposition problem and visualize the results. A case study is presented on the decomposition of the BIW substructure consisting of side frames and floor panels for minimum distortion under global torsion. Representative optimal designs are selected and the trade-offs among stiffness, manufacturability, and assembleability are discussed.