The effects of addition of Sc on the aging behavior of Mg–Gd alloy was examined by means of Vickers microhardness measurements, transmission electron microscopy (TEM), and high-resolution TEM. The maximum hardness of the specimens increased with increasing Sc content. In the early stages of aging, contrasts of precipitates that were similar to the β'' phase (D019-type structure) in binary Mg–Gd alloy were observed in the Mg–Gd–Sc alloys, whereas specimen that had started to harden again, but had not attained maximum hardness, showed predominant contrasts for precipitates that were similar to the β' phase (base-centered orthorhombic structure) in binary Mg–Gd alloy. Contrasts for precipitates with a D019-type ordered structure appeared during prolonged aging in specimen with high contents of Sc. Therefore, the addition of Sc to Mg–Gd alloys tends to slow down the transformation from the '' phase to the ' phase.