For severe frontal and side impact loads, the passenger compartment of small buses does not retain its initial geometry, and plastic deformations are inevitable. Therefore, linear analysis alone is insufficient for the analysis and an elasto-plastic analysis technique for evaluating the structural strength capacity and the inelastic deformation characteristics must be considered. A three-dimensional mathematical model of the passenger compartment was prepared using the finite element simulation method. A three-dimensional elasto-plastic structural analysis was developed along with a computer program, named EPSAP, to evaluate the structural strength capacity of the passenger compartment under quasi-static loading conditions. The procedure provides a technique for: (1) tracing the load-displacement history of a space frame structure, (2) determining the collapse or the ultimate load, (3) predicting the collapse mode, and (4) evaluating the energy absorption capacity.