The microstructure and Vickers hardness of the Mg97Zn1Y2 RS P/M alloys extruded at several temperatures (573Ksimilar to723K) have been investigated. Transmission electron microscopy (TEM) observations indicated that the 6 periodic hexagonal (6H) structure was formed in these RS P/M alloys. With increasing the extrusion temperature from 573 to 723K, the average grain size increased from 180 to 490nm. The Mg grains of the Mg97Zn1Y2 RS P/M alloy consolidated at 723K grew form 490nm to 1.3 mu m, in diameter by annealing at 773K for 5h. However, the 6H structure existed without any apparent structural change due to its high thermal stability. The Vickers hardness decreased from 136 to 69 with increasing of the grain size. However, the Vickers hardness of the Mg97Zn1Y2 RS P/M alloys estimated on the Hall-Petch relation was much higher than that of pure Mg. The results indicated that a remarkable gap between the Hall-Petch relations of the Mg97Zn1Y2 RS P/M alloys and pure Mg was originated from 6H structure in the Mg97Zn1Y2 RS P/M alloys. The present results demonstrate the 6H structure improved the strength of the Mg97Zn1Y2 RS P/M alloys as well as the fine grains.