This paper introduces side impact of an auto-body with reinforced with TRIP steel for the purpose of safety enhancement and weight reduction. Cowper-Symonds model and Johnson-Cook model have been applied to the analysis in order to investigate the strain rate effect compared to the quasi-static model. The analysis has made TRIP steel supersede the conventional low carbon steel for center pillar and side sill of auto-body side structures. Side impact analysis has been carried out based on the condition in FMVSS 214. The numerical simulation has been performed with LS-DYNA3D in order to investigate the effect of TRIP steel on the dynamic response in side impact as well as the strain rate hardening effect. Analysis results show that auto-body structures reinforced with TRIP steel are desirable for weight.