Abstract: Hot compressive deformation behavior of the AZ91 magnesium alloy performed at the temperature of 250-400 ℃ and the strain rate of 10-3-1 s-1 was examined by Gleeble-3500 thermal mechanical simulator. It was shown that the AZ91 magnesium alloy had completely dynamic recrystallization during hot compressive deformation at the temperature of 350-400 ℃ and the strain rate of 10-3-10-2 s-1, obvious crack was not found on the alloy surface. The average grain size was smaller than 10 μm when the strain was 70% at 350 ℃ and 10-2 s-1.