In this study, the hot deformation behavior of a magnesium alloy was investigated under various process conditions. Tensile testing experiments were performed to determine the effects of temperature, strain, and strain rate on the flow stress of the material. A new constitutive model was established to characterize the dynamic recrystallization of the magnesium alloy at elevated temperatures. The critical strain was evaluated based on the temperature-compensated strain rate to consider the work softening. The amount of high temperature softening due to dynamic recovery and dynamic recrystallization was formulated as a function of strain, strain rate, and temperature. It was demonstrated that the proposed model is able to predict the flow softening as well as the growing strain hardening of the material very accurately. The failure characteristics were also studied at different temperatures and strain rates. Finally, the grain size effect on the flow behavior of the material was discussed.