The main load-supporting elements for an automobile framework are thin-walled beams. To study the effectiveness of lightweighting on the body stiffness, the analytical formulae describing static bending and torsional stiffness of a beam with fixed-fixed supporters were derived firstly. To get a light thin-walled beam structure with optimum section, the influence of every sectional geometry parameter on the stiffness-mass ratio was discussed respectively. After replacing the steel by the lightweight material, the corresponding variations in structural stiffness and size were analyzed. The stiffness-mass ratios of beams with open and close loop sections were also compared with each other. Finally, a few geometrical and material parameters, which can greatly influence the bending or torsional stiffness of these thin-walled beams, were found out. For a typical thin-walled beam applied in an automobile body, the design guideline achieving both lightweight and high stiffness is presented.