Sand-cast Mg-9Gd-4Y-0.5Zr (wt％) alloy was solution-treated at 500-565 ℃ in the time range of 0.5-30 h in air or vacuum to investigate its microstructure evolution and mechanical properties.The results showed that solution treatment temperature had a significant influence on the dissolving rate of eutectic phase and grain growth.Taken both of them into consideration,510-520 ℃ was considered to be the optimum solution treatment temperature range for this alloy.It should be noted that the trace (0.4-0.9 vol％) and insoluble cuboid-shaped phase precipitated during solution treatment was identified to be YH2,of which the hydrogen was thought to come from both the melting and solution heating process.In addition,the 3D morphology and dissolving process of Mg24(Gd,Y)5 eutectic phases in the as-cast alloy were also discussed via in-situ observation under X-ray tomography.