With the surge of environmental problems, the number of small vehicles with lower energy consumption is increasing. For such a vehicle, difficulty of achieving crashworthiness exists since it has smaller deformation space in a frontal impact accident. Smaller deformation space needs high vehicle deceleration for absorbing kinetic energy of the vehicle.The high vehicle deceleration in the event of a frontal collision may produce high occupant deceleration, resulting high chances of an occupant injury value.In this study, over 138 types of vehicle decelerations expressed by three variables in a frontal collision were examined in order to reduce occupant decelerations. Solving differential equations, following results were obtained (1)The minimum value of the maximum occupant deceleration exists. The value is 45.2% lower than the value whose deceleration steps up gradually.(2)The vehicle deceleration that produces the minimum value of the maximum occupant deceleration is confirmed to include acceleration; i.e. initially the vehicle decelerates, next accelerates, and then decelerates.(3)The most optimum way of countermeasure that reduces the maximum occupant deceleration become clear. If an available amount of changing vehicle deceleration is small, the best way to reduce the maximum occupant value is different from the way to the most optimized value of the maximum occupant deceleration.(4)One of the ideas of actual vehicle structure that is able to make the deceleration be minus value in frontal collision is presented.