Hot compression tests of die-cast AZ91D magnesium alloy were performed on Gleeblle-1500D at strain rates ranging in 0.01~10 s-1 and deformation temperatures ranging in 250~450 oC. The constitutive equation, =1.41×1012 [sinh(0.014σ)]5.295exp (–159 449.509/RT), which is described by Zener-Hollomom parameters for hot compression deformation of die-cast AZ91D magnesium alloy was established. Compared to gravity cast AZ91D magnesium alloy, the deformation activation energy of die-cast AZ91D magnesium alloy, Q=159.45 kJ·mol-1, determined by hyperbolic sine function is lower