Based on dislocation theory,we investigate the mechanism of strain rate effect.Strain rate effect and dislocation motion are bridged by Orowans relationship,and the stress dependence of dislocation velocity is considered as the dynamics relationship of dislocation motion. The mechanism of strain rate effect is then investigated qualitatively by using these two relationships although the kinematics relationship of dislocation motion is absent due to complicated styles of dislocation motion.The process of strain rate effect is interpreted and some details of strain rate effect are adequately discussed.The present analyses agree with the existing experimental results.Based on the analyses,we propose that strain rate criteria rather than stress criteria should be satisfied when a metal is fully yielded at a given strain rate.