An experimental study on hot mechanical behaviors and microstructure evolution of the commercial magnesium alloy AZ31D is presented in this paper. The main experimental works in this paper include hot compression tests of cylindrical specimen at different temperatures and strain rates and microstructures observations of the deformed specimens. The experimental results show that the smallest of dynamically recrystallized grain size appears at 250 degrees C and at 10(-1)s(-1), and the maximum volume fraction of dynamically recrystallized is obtained at 350 degrees C and at 10(-3) s(-1). The dynamic recrystallized grain size in above conditions is about 10 mu m. According to the experimental results a set of ECAE tests for grain refinement is designed and carried out at 300-350 degrees C.