Abstract: AZ91D semisolid billets were prepared by a self-designed wavelike sloping plate device. Mechanical behavior and microstructure evolution of isothermal compression deformation in AZ91D semisolid billets have been studied using a Gleeble1500 thermal simulation machine and optical microscopy. When deformation temperature was constant, true stress increased with increasing strain rate; when strain rate was constant, true stress decreased with increasing deformation temperature. The deformation mechanisms of AZ91D alloy were quasi-elastic deformation during isothermal compression and phase interface sliding under thermal activation. The microstructures of isothermal compression of AZ91D semisolid billets consist of globular and near-globular grains that are caused by the round-grinding of α-Mg grains under shear stress and Ostwald ripening under surface tension.