Effects of various aging treatments on microstructure and mechanical properties of the Mg–4.5Zn–4.5Sn–2Al alloy are comparatively studied by OM, SEM, TEM, tensile testing and micro-hardness measurement. Two hardness peaks are discernible in all the age hardening curves, corresponding with the precipitation of β1′ followed by the precipitation of Mg 2 Sn. The first peak hardness is lower than the second peak by single aging at both 150°C and 200°C. The peak hardness can increase to more than 90HV by single aging at 150°C and double aging. Double aging brings about the relatively steady hardness beyond 87HV with the second aging at 200°C from 5h to 50h. The long rod-like β1′ phase, the short rod-like Mg 2 Sn phase and the two-phase T-shaped particles are dominant in the double aged sample. Double aging brings about the obvious refinement of precipitates and the enhanced the nucleation of the Mg 2 Sn particle on the tip of the β1′ long rod. The double aged alloy exhibits the highest strength and the biggest elongation among all the as-aged alloys, with the strength comparable to that of single aging at 150°C for 70h and the plasticity about 27% higher. The strength improvement is attributable to the synergy effect of the refinement of precipitates, the formation of the short rod-like Mg 2 Sn precipitates with larger aspect ratios and the enhanced formation of the two-phase T-shaped particles.