The effect of heat treatment on the microstructural and tensile properties of an ultrasonically treated extruded Mg-6Zn-0.5Y-2Sn alloy was investigated. The results showed that after both the aging treatment and “solid solution and aging” treatment, the grain size became more uniform. The MgSnY and Mg2Sn phases were much more dispersed and increased in amount. In addition, both the aging and “solid solution and aging” treatments improved the yield strength of the test alloys. Specifically, the aging treatment significantly improved the yield strength from 165 MPa to 269 MPa, which is an increase of 63.1 %. The precipitation strengthening, which is derived from the collective strengthening effect of the MgSnY and Mg2Sn phases, was the optimal factor for improving the yield strength in the test alloys with heat treatment. Moreover, the aging treatment more effectively improved the comprehensive tensile properties of test alloy compared with the “solid solution and aging” treatment. The mechanisms for the experimental observations were also discussed in this research.