Abstract: The hot compression behaviors of AZ31 magnesium alloy at 250~400℃ and a strain rate of 0.5~3,0 s^-1 were investigated by means of isothermal compression test on a Gleeble-1500 thermal mechanical simulator. The true strain-true stress curves of AZ31 alloy under different deformation conditions were obtained, The microstructure after deformation was observed by optical microscopy. The results showed that dynamic recrystallization was a main mechanism responsible for grain refinement under the test conditions and was influenced by deformation parameters.