The effect of heat treatment on microstructure and mechanical properties of sand casting Mg-Gd-Y system alloys were investigated systematically by optical microscopy(OM), scanning electron microscopy(SEM) and tensile tests. The results showed that the microstructure of solutionized GW94, GW74 and GW44 Mg alloys was mainly composed of supersaturated α-Mg solid solution, residual phases of Mg₅(Gd,Y) as well as square phases formed during solution treatment.As the content of Gd addition to Mg-Gd-Y alloys increased, the volume fraction of square phases in the matrix increased. The time of reaching peak hardness decreased in the same aging temperature. The ultimate tensile strength and yield strength increased gradually (the yield strength of solutionized alloys firstly decreased and then increased), while the elongation-to-failure decreased gradually from solution treated and peak-aged alloys at room temperature to peak-aged alloys at 200 and 250℃. The peak-aged GW94 Mg alloys exhibited superior mechanical properties.The ultimate tensile strength, yield strength and elongation-to-failure of the peak-aged GW94 Mg alloys at room temperature were 300,7 MPa and 0.9%.The ultimate tensile strength, yield strength and elongation-to-failure of the peak-aged GW94 Mg alloys at elevated temperature (200 and 250℃) were 329,4MPa, 2.6% and 312,3MPa, 2.7%, respectively. The peak-aged GW94 and GW74 Mg alloys showed a abnormal behavior that the ultimate tensile strength increased with the increase of temperature.