Occupant and occupant restraint system were modeled mathematically using multi-body dynamics. Three dimensional models of HYBRIDⅢ, 3-points seat belt with energy dissipation and slip, car interior system and energy absorbing steering system were described. In this paper, DADS(Dynamic Analysis and Design System) was used to simulate the occupant behavior in sled tests and barrier tests. Simulations of sled tests were conducted using DADS and results were compared with real tests and MADYMO3D simulations. Modeling of frontal impact was also made for a passenger car and the various injury parameters were investigated. The effect of parameters associated with seat belt(shoulder belt slack, lap belt slack, belt stiffness, retractor performance and outer D-ring position) on occupant injury was investigated and the seat belt design was made to increase safety using orthogonal array and ANOVA.