On equality with chest deceleration, it is a very important subject to reduce chest deflection when frontal crash impact of vehicle occurs. To resolve the principle of chest deflection occurrence, first of all we calculated the spring constant and damping constant between a front rib and a spine of Hybrid III dummy by analyzing the thorax impact calibration test. Next we calculated the chest deflection of the dummy spring-damper-mass model being pressed external force by using convolution integral. We also conducted the three patterns sled test. Compared between calculated value and tested value, both values have relatively nearly good compatibility as for the peak value of chest deflection. According to the result, we also noticed that it has a close relationship between chest deflection and seatbelt force.