Variation of S-parameter with the strain [S（ε）curves] under the strain rate of 5×10 5/s in high purity iron was investigated by Doppler broadening measurements combined with slow tensile tests. In order to estimate dislocation density pd and vacancy concentration Cv

the deformed samples were annealed at o00K for 10h to eliminate the effect of vacancies induced by the strain and to retain the strain induced dislocations.A calculation based on the trapping model gave the dislocation density ρd on the order of 109-1011cm2 and the vacancy concentration Cv.on the order of 1017-1019cm3.both of which were increased with the strain.