The upsettability test and backward extrusion are carried out to investigate forging limit of a commercial AZ31 magnesium alloy (Mg–3%Al–1%Zn) with controlled grain size at temperatures of 293–673 K. To change the grain size of the AZ31 specimen, the specimens are annealed at temperatures of 473–773 K before forging. By annealing at higher than 673 K, the grain size of the specimen becomes large, and the anisotropy of the specimen is reduced. In forging at lower than 473 K, forging limit without fracture of the specimen annealed at 773 K is higher than that of the specimen annealed at 473 K. On the other hand, in forging at higher than 473 K, the forging limit of the specimen annealed at 773 K is lower than that of the specimen annealed at 473 K. Thus, it is considered that the refinement of the grain size is effective on the forging limit in warm forging, and the controlling of the anisotropy of the specimen is effective on the forging limit in cold forging.