Mg–Al and Mg–Zn alloys containing Si or Ca, which forms intermetallic compounds having high melting temperatures with magnesium, were initially strained compressively and then press-formed at the semi-solid state. Microstructures, heat treatment characteristics and mechanical properties of the formed specimens were evaluated. Microstructures with uniformly distributed fine and spherical particles are obtained by applying the semi-solid forming to the strain-induced bulk. The Mg–Al specimens with Si or Ca have microstructures with the fine eutectic phase produced by the rapid cooling and exhibit precipitation structures in the whole grains, resulting in the marked age hardening. As the forming temperature rises, the amount of liquid phase increases and the fluidity of a semi-solid material is improved, resulting in the decreased generation of defects during forming. But the mechanical properties become degraded due to the increased amount of the eutectic phase. To increase the forming temperature also causes the localized liquid phase to the edge side of the disc-like specimens and results in the formation of the non-uniform macrostructures. Tensile properties of all formed specimens at the ambient temperature are far superior to those of the as-cast specimens, but lower than those of the formed AZ91D magnesium alloy. The tensile properties, however, become superior to those of the AZ91D magnesium alloy at temperatures above 150°C. In particular, the Mg–Al–Ca alloy shows high strength at elevated temperatures because of the fine Al₂Ca phase crystallized at grain boundaries.