In order to study the warm forming strengthening behaviours of AZ31 magnesium alloy, plane strain compression tests were carried out at the temperatures ranged from 210 to 330°C for as-cast and pre-forged Alloys. Tensile specimens were machined from the compressed samples. Room temperature mechanical testing was conducted according to the specifications. Microstrutres were investigated using scanning electron microscope. The analysis revealed that there is an excellent warm forming strengthening effect for as-cast AZ31 alloy with hexagonal close-packed structure. The increments of the tensible strength is about 40～50%. With decreasing temperature, the tensible strength increases during warm forming. The deformation extent and deformation mode has its different influence on strengthening effect. The high strength was attributed to the fine-grain strengthening and work hardening. This suggests that fine grain and high strength are attained for AZ31 alloy by means of rational combined action of work hardening and fine-grain strengthening during warm forming.