The dynamic recrystallization refinement of magnesium alloy AZ80 by compression tests was studied, and its effect on the mechanical properties was investigated. It is observed that the microstructure of the as-cast billet with grain size of 240 μm becomes refined to about 120, 110, 94 and 50 μm after upsetting at 350 ℃ under strain rates of 0.01, 0.1, 1 and 10 s-1 respectively. The changes in the mechanical properties according to grain size show that yield strength significantly decreases with grain size increasing, while strain hardening exponent and micro hardness increase very sharply. Further, the grain size vs strain rate and change in Vickers micro hardness according to the various strain rates show that grain size and micro hardness decrease with strain rate increasing.