In order to investigate the strengthening effect of manganese dispersoid in manganese-added Al-Zn-Mg alloys, specially designed alloys with various manganese content were prepared and evaluated. The manganese dispersoid in the alloy is found to increase the strength significantly without losing much elongation. The strengthening effect originates from the fact that the manganese dispersoid behaves as the non-shearable particle which is composed of Al, Mn and Zn. It can be concluded that the improvement of mechanical properties by the manganese dispersoid without losing much elongation is due to the strengthening effect produced by the pinning action of the dispersoid on dislocation glide and the enhancement effect in elongation caused by a homogenization of slip. Meanwhile, the manganese dispersoid and soluble manganese element in the peak-aged manganese-added Al-Zn-Mg alloys are observed not to influence the width of the precipitate-free zones (PFZ) and the kinetics of precipitates such as eta’ and GP zone (solute rich duster).