In order to evaluate dynamic fracture toughness of structural steel, three point bending (3 PB-COD) or compact tension (CT) test is usually carried out by using drop weight or high tensile rate testing machine. Since, in static test, applied load to the specimen can easily measured exactly by load cell of test system, but in dynamic test it is difficult to get or decide working load on test specimen because the effect of acceleration affects on the output of load cell in test system. In this paper, the estimation of working load on test specimen was investigated applying a simple mass and spring model to the loading system of test equipment.