Stress-strain curves of some kinds of materials at high strain-rate conditions were able to be determined by a drop weight experiment system which has only to measure the force-time relation using a load-cell but not to measure directly the deformation or deformation-rate of specimen. To evaluate the strain-rate or the strain of the specimen it had been necessary to measure the motion, i.e. the velocity or the displacement of tup and anvil so far. In this new method the velocity and the displacement of the tup and the anvil which contacted the both end surfaces of specimen were calculated using a personal computer on the basis of the equations of motion for the tup and the anvil, respectively. The differential equations, in which the measured dynamic-force versus time characteristics were contained, were integrated by Runge-Kutta method using the personal computer. In the differential equation of motion of the anvil, a spring coefficient K for the rubber cushion beneath the anvil is used. For the first approximation of the coefficient K is assumed to be the value determined by the oscillation method of cantilever beam. The spring coefficient K with high accuracy is determined when the computed strain of the specimen on the basis of the method described above is almost equal to the measured strain of it by using a micrometer caliper after the dynamic compression. The coefficient K with the higher accuracy can be obtained the incremental compression experiment using some kinds of hard stop ring in the prescribed height.