The post-dynamic softening of Nb micro-alloyed steel and plain carbon steel was investigated through stress relaxation method. Meanwhile, By comparison with C-Mn steel, the effect of deformation temperature and strain on post dynamic recrystallization of Nb steel was studied . The result shows that element Nb in solution state can dramatically improve the apparent activation energy of static recrystallization. At lower temperature, the recrystallization can be retarded and precipitation of Nb(C, N) occured. If strain is more than the critical strain of dynamic recrystallization, the metadynamic recrystallization takes place. If fully metadynamic recrystallization takes place, the kinetics is no longer dependent on strain of the kind of steels.