Microstructures and Vickers hardness have been investigated in hot-rolled Mg-3mass%Y based solid solution alloys containing microalloying elements (Ca, Ag, and Ni). Transmission electron micorscope (TEM) observations have revealed that the stacking faults on the (0001) magnesium matrix planes have been observed in Mg-Y-Zn based alloys and the stacking fault (SF) density depends on other additional microalloying elements. In single addition of Zn to the Mg-Y alloy, SF density increases with increasing Zn content and was saturated over 0.5 mass% addition. On the other hand, in simultaneous addition of Zn and Ca, SF density increases with increasing Ca content significantly. Many precipitates were observed in Ni and Ag added Mg-3Y-0.5Zn alloys and their SF densities were lower than Mg-3Y-0.5Zn. Vickers hardness increased by the simultaneous microalloying of Zn and Ca, while Ag showed a negative effect for hardness in Mg-3Y-0.5Zn (in mass%) ternary alloy. The dense SF density could act as obstacles to the dislocation motion so that SF density has positive relationship in the Vickers hardness.