In this paper, collapse analysis is performed under the quasi-static loading condition using finite element limit analysis to grasp the collapse behavior of an auto-body side structures according to the change of design variables at the initial stage of design. The finite element limit analysis is useful to determine a proper distribution along center pillar axis of design parameters like sections' shape or thickness. The collapse load and the collapse mode are calculated for the standard model and the analysis executes changes in design based on the result from the standard model, to enhance the energy absorption efficiency and perform an S-type deformed shape for limiting injuries at thorax level. This enhancement can be achieved by increasing the collapse load and by controlling the deformed shape with reinforcing weak parts of the side structure. The optimum design of an auto-body structure needs high load carrying capacity and a proper deformation of center pillar to guarantee passenger's safety.