Variation of S-parameter with the strain (S-ε curve) under different strain rate are investigated by combining Doppler broadening measurement with slow tensile test. Experimental results are as follows:1. Under slow strain rate, the S-ε curve in high purity iron does not tend to saturation. The slower the strain rate is, the more evident the "non-saturation phenomenon".2. Variations of S-ε curve with strain rate are attributed to the different configuration of dislocation distribution under different strain rates.3. As high purity iron is deformed, the increment of S-parameter depends not only on dislocation density but also on vacancy concentration, and we attibute the so called "non-saturation phenomenon" of S-ε curve to the strain induced vacancies.