This paper investigated the effect of aging treatment before extrusion on microstructure and mechanical properties of AZ80 magnesium alloy. Fractural microstructures of the test specimens were also analyzed by scanning electron microscopy. Results show that aging treatment before extrusion can remarkably promote grain refinement; the dispersed Mg₁₇Al₁₂ particles that precipitated from the matrix during aging treatment before extrusion distribute in the grain boundaries, during the subsequent extrusion process these particles can pin grain boundaries migration and lead to fine microstructure; however, with prolonging the aging time, the grain refinement effect was weakened. After aging treatment and subsequent extrusion, the yield strength, tensile strength and elongation are increased. Based on fracture surface analyzed, the premature cracks generate around big particles that present in grain boundaries lead to loss of elongation. The strengthening effect of aging before extrusion in AZ80 magnesium alloy proven in this paper provides a new method for the design of the relatively high-performance magnesium alloys.