There are two fundamental techniques that can be employed to improve side impact protection. One approach involves increasing the reaction force of the vehicle body in the interval between the onset of the crash (primary impact) and the moment the door strikes the occupant (secondary impact) The second approach involves improving the energy absorption capacity of the door inner assembly at the time of the secondary impact. This work focused on the first technique of improving the body reaction force. Using a dynamic nonlinear FEM simulation program (PAM CRASH), an analysis was made of the body deformation behavior of a four-door sedan in a side impact. In conducting this analysis, a method was developed for preparing an analytical model having sufficient size to provide good correlation with the deformation behavior of an actual vehicle in the interval between the primary and secondary impacts. The results obtained with the model were then analyzed by examining time histories of the displacement of body structural elements and strain distributions during the side impact phenomenon. The purpose of the analysis was to find ways of reducing the impact velocity between the door and the occupant, which would be most effective in mitigating the thoracic trauma index (TTI) of the occupant. The results of the analysis clarified the deformation mechanism of the body structures and indicated the respective contribution that various body reinforcements would make toward increasing the reaction force of the vehicle body.