The effect of hot extrusion and aging treatment on microstructure and mechanical property of Mg-6Zn-1Zr-1.5Y alloy was investigated and the strengthening mechanism was clarified. The elongation increased and the strength first increased and then decreased with increasing extrusion ratio. Mg3Y2Zn3 phase precipitated during hot extrusion and the dislocation density decreased during aging treatment. The strength and elongation of the extrusion bar (λ=10) decreased, the strength and elongation of the extrusion bar (λ=13) increased, and the strength increased and the elongation decreased of the extrusion bar (λ=25). The major strengthening mechanism is aging strengthening, strain-hardening and Hall-petch strengthening. The mechanical property depended on Hall-petch strengthening and strain-hardening after hot extrusion, and depended on aging strengthening and strain-hardening after aging treatment