The flow stress behavior and microstructural evolution in as rolled AZ91 alloy were studied by tensile test at temperature ranging from 573 K to 698 K and with initial strain rate of 10-3-1s-1 .The results show that the activation energy of the AZ91 alloy is 96 kJ/mol and the stress exponent is 3.5.Grain refinement is observed during hot deformation.It is found that grain refinement is due to dynamic recrystallization,but grain size is decreased with increasing Zener-Hollomon parameter.The deformation mechanism is dislocation cross-slip,dislocation climbs and twin intersections,which are probably suitable to the development of substructures.Twins always exist in matrix whether they are at high temperature or at low temperature.Strain induced precipitation of Mg17 Al12 occurs in the AZ91 alloy and stabilizes the subgrain boundaries.