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The microstructures of Mg96.17Zn3.15Y0.50Zr0.18 alloys solidified under 2-6 GPa high pressure were investigated by employing SEM (EDS) and TEM. The strengthening mechanism of experimental alloy solidified under high pressure is also discussed by analyzing the compressive properties and compres-sion fracture morphology. The results show that the microstructure of experimental alloy becomes significantly fine-grained with increasing GPa level high pressure during solidification process, and the secondary dendrite arm spacing reduces from 40μm at atmospheric pressure to 10μm at 6 GPa pressure. The morphology of the second phases changes from the net structure by the lamellar-type eutectic structure at atmospheric pressure to discontinuous thin rods or particles at 6 GPa pressure. Besides, the solid solubility of Zn in the Mg matrix is improved with the increase of the solidification pressure. Compared with atmospheric-pressure solidification, high-pressure solidification can improve the strength of the experimental alloy. The compressive strength is improved from 263 to 437 MPa at 6 GPa. The fracture mechanism of the experimental alloy changes from cleavage fracture at atmospheric pressure to quasi-cleavage fracture at high pressure. The main mechanism of the strength improvement of the experimental alloy includes the grain refinement strengthening caused by the refinement of the solidification microstructure, the second phase strengthening caused by the improvement of the morphology and distribution of the second phases, and solid solution strengthening caused by the in-crease of the solid solubility of Zn in the Mg matrix.