Occupant and occupant restraint system were modeled mathematically using multi-body dynamics. Three dimensional models of HYBRID lll, 3-points seat belt, car interior system and energy absorbing steering system were described. Occupant behaviour simulations in sled tests were done usig DADS(Dynamic Analysis and Design System) and compared with real tests and MADYMO3D simulations. Occupant behaviour simulation in frontal impact were also done for a passenger car and various injury parameters were investigated . The effect of parameters associated with seat belt on occuptant injury was evaluated and the seat belt design was made to increase safety using orthogonal array and ANOVA.