The dynamic stress-strain behavior of the AZ91 alloys in different treatment conditions (as-cast, T4 and T6) was investigated by means of split Hopkinson pressure bar. It was found that the flow stress increased at first, and then declined with the strain rate increasing at the range of 102~103s-1 for the alloys in these three conditions. And the alloys exhibited both positive and negative strain rate effects. The former was caused by strain rate strengthening and the latter was caused by strain rate weakening. However the flow stress for the alloy in aged condition at the same strain rate was higher than both of the alloys in as-cast and solution conditions. The study also showed that the maximum strains of the alloys in different conditions increased with the strain rate increasing, and the strain rate to fracture for the alloy in solution condition was higher than those of other two alloys. The work-hardening of α–Mg matrix and the reinforcement of β-Mg17Al12 phases led to the strengthening of the alloy, while thermal softening of matrix, the fracture of β phases and initiation and propagation of the cracks were responsible for the weakening of the alloy.