A study is presented on the effectiveness of front underrun protector (FUP) for Heavy Duty Vehicles based on the analyses of accident investigation data, full scale front underrun tests on passenger car, and computer simulation. From accident investigation data, the magnitude of cars underrun travel where fatal or serious injury occurs is shown. Also we show the relation between collision speed and the car underrun travel during the accident. This relation can be shown by the approximate formula with a parameter of the overlap ratio of the car width when colliding. For the final step, the effectiveness of PUP were estimated by computer simulation based on the results of full scale tests when the strength and the ground clearance of FUP were variously changed.