Stress relaxation tests at various strain rates and the repeated stress relaxation tests were carried out at room temperature to obtain the internal and effective stresses as well as the Gupta-Li's stress exponent, m*, in NaCl single crystals. It was found that the constant, a, appearing in the time pharenthesis of the integral form of the Gupta-Li equation, decreased with an increase of the strain rate accompanied by the increase of effective stress. With increasing cycle number of repeated relaxation experiment, it was found that the effective stress decreased monotonously, whereas the internal stress increased. The repeated relaxation curves cannot, in general, be superimposed on the initial stress relaxation curve if the re-strain rate is changed. This indicates that the mobile dislocation density (which is considered to be constant during one cycle of testing period) and the effective stress during the repeated stress relaxation test are not altogether equal to those of the initial stress relaxation test. However, if the mobile dislocation density could be chosen to be the same as that at the initial stress relaxation test, it may be possible to superimpose these two relaxation curves by translating along the time axis to the point where each effective stress agrees. Therefore, if the re-strain rate is too small not to produce the mobil dislocation density comparable with that in the initial test, the superposition is not to be expected. This indicates that the increasing rate of mobile dislocation density as a function of strain depends on strain rate.