Compatibility of vehicles of different mass and stiffens in head-on collisions is studied in this paper with the aid of an eight-degrees of freedom, two dimensional lumped-mass simulation model. The model takes into consideration mass and stiffness ratio as the main factors contributing to compatibility of the two colliding vehicles. Other factors like length of crumple zone, offset overlap and speed are also considered. Three injury risk criteria have been considered in this study;​ delta V or change in velocity of vehicle after impact, maximum acceleration sustained by the passenger compartment throughout impact, and length of deformation sustained by the car front. The most crucial compatibility parameter is the mass ratio for the delta V criterion. The second and third compatibility parameters are mass ratio and stiffness ratio for deformation length criterion. A longer crumple zone is proposed for heavier vehicles to provide the required protection for smaller vehicles involved in head-on collision. It was found that both mass and stiffness ratio have no aggressivity on the partner vehicle when maximum acceleration criterion is used at low impact speeds.