Stress relaxation of NaCl single crystal was measured by the method using transmission of polarized light. It became possible by this method to analyze the time variation of stress in the direction perpendicular to the compression axis. It was found that the mechanism of relaxation of the principal stress [σ2] in short time range was different from that in long time range. The initial process of stress relaxation was strongly influenced by the strain rate of compression. This influence was found to be strong but continue for a short time when the strain rate was high, and weak but for a long time when it was low. It was observed that σ2 decayed following the Maxwell type viscoelastic model at the early part of the initial short time range of the stress relaxation, and thereafter σ2 increased gradually by the pinning of moving dislocations and finally attained at a constant value.