A356 alloy was processed by semi - solid squeeze casting followed by solution treatment at 540℃. With increasing solution time, Mg and Si elements dissolved into the matrix and led to solid solution strengthening. Tensile strength, elongation and hardness peaked at 6 h and then decreased during solution treatment. Aging treatment at 180℃ for different time was carried out after solution treatment. With increasing aging time, the fine spherical Mg2Si phases precipitated in the A356 alloy matrix, which were ca. 2 μm in size. The optimum heat treatment process was solid solution at 540℃ for 6 h followed by aging at 180℃ for 4 h. After solution and aging treatment, tensile strength and elongation reached 336 MPa and 6.9%, respectively. Hardness value reached 124 HV after heat treatment, which was 106.7% higher than the cast alloy.