In contrast to the typical behavior at macro level,recent studies at sub-micro scale have shown that metals may harden rather than soften during high temperature annealing.In the present work,a modified three-dimensional discrete-continuous mode(DCM)is proposed to address the limitations of the local model,in which the vacancy diffusion equation is solved by the FEM.Based on this DCM model,the intrinsic mechanism of annealing hardening at sub micro scale and detailed evolution of defects during the annealing are investigated.Numerical simulation results shows that the hardening is caused by the decrease of dislocation density,which can be ascribed to two main aspects:(i)The climb of dislocation during the high temperature annealing promotes the annihilation of the dislocations,leading to a decrease in dislocation density.(ii)The jogs,nucleated during annealing due to the high temperature climb,have very weak mobility and act as obstacles to the dislocation glide motion,leading to mobile dislocation starvation and resulting in a higher flow strength level.