Due to the ever-challenging demand for an improved fuel efficiency and enhanced environmental regulations, weight reduction for commercial vehicles has been one of the hottest issues in the automotive field. In this paper, lightweight design for a bulk trailer is performed using topology optimization. For FE analysis, a trailer model is simplified with the removal of axle, suspension arm, and the fifth wheel. Three extreme driving conditions are considered as boundary conditions. Compliance minimization with a mass constraint is formulated in order to determine the lightweight design under the given boundary conditions. A pilot model is manufactured based on the optimized design through discussion with field experts. By virtue of design optimization, the pilot model can achieve a mass reduction of 14% compared with the original model.