Si-enhanced AZ91 magnesium alloy was obtained through Rapid Solidification/Powder Metallurgy (RS/PM). The effects of different Si content on the microstructure, the room and elevated temperature mechanical properties of AZ91 alloy were examined. The results show that Mg2Si particles generated in situ grew up gradually as the increasing of Si content. Adding Si to the alloy can significantly improve the room and elevated temperature mechanical properties. At the room temperature, the tensile strength of alloys improved with the Si contents increased when the Si contents less than 3(mass)%. But when the Si contents increased to 5(mass)%, the alloy tensile strength reduced significantly. The alloy with 3(mass)%Si exhibits the best mechanical properties, its 0.2% yield tensile strength, ultimate tensile strength and elongation of rupture were 354.93 MPa, 472.36 MPa and 4.70%,respectively. And the alloy elevated temperature (473 K) tensile properties improved as the Si addition increased.