Abstract: Compression deformation and semi-solid isothermal heat treatment of Mg-3Al-0.8Gd alloy were carried out. The effects of the compression deformation (10%,15%,20%), isothermal temperature (530,540,550,560,570 ℃) and holding time (3,5,10,15 min) on the microstructure and hardness of the alloy were studied. The tensile and impact properties of as-cast and heat-treated Mg-3Al-0.8Gd alloys were compared. The results show that the microstructures of Mg-3Al-0.8Gd alloy after compression deformation and isothermal heat treatment under different conditions were all composed of α-Mg matrix and β-Mg₁₇Al₁₂ phase. With the increase of compression deformation, isothermal temperature and holding time, the α dendrites were gradually transformed into equiaxed grains, the grains were refined, the structure uniformity was improved, and the microhardness increased. The tensile strength, elongation after fracture, section shrinkage and shock absorption energy of Mg-3Al-0.8Gd alloy with compression deformation of 20% after isothermal treatment at 550 ℃ for 15 min were 11.3%, 32.6%, 3.8% and 23.3% higher than those of the as-cast alloy, respectively.