Lightweight has been the main focus of the auto industry in recent years. To improve the competitiveness, the weight reduction should be achieved without compromising the performance. A new lightweight strategy of auto body is proposed in this research to improve the efficiency of the traditional lightweight method. Topology optimization is first applied to find the weak part of the BIW (Body-In-White) and small size parts are added to improve the overall performance. Punching and shape cutting are also applied in this step to reduce the material area. This will maximize the potential for the later lightweight. Sensitivity analysis is then applied in this study to optimize the plate thickness. By optimizing the thickness, the weight of the BIW reduced significantly. The various performance verifications were carried out to verify the efficiency of the optimization. The results indicate that the lightweight is achieved successfully while keeping the main performance uncompromised. The combination of the topology optimization and sensitivity could maximize the potential for the lightweight and is of great effectiveness in weight reduction. It also serves as a good example for other auto parts lightweight problems.