The effects of nln dispersoids on the the enhancement of mechanical properties in Al-Mg-Si(-Mn) alloys have been studied to develop a new high Mn alloy which does not need an aging heat treatment after a shaping process (i.e. extrusion process). By adding dirt to Al-Mg-Si alloys, sphere- or rod shaped Mn dispersoids of a size ranging from 0.05 to 0.5 mu m are formed by the use of proper heat treatments. The as extruded alloys containing 1.0 wt-%Mn are measured to have higher tensile properties with good ductility, as compared with those of the commercial Al alloy 6N01 (Al-0.69Mg-0.79Si-0.48Cu-0.27Zn-0.37Mn-0.3Cr-0.11Ti, wt-%). These phenomena are obtained from the dispersion hardening effect and homogeneous deformation by Mn dispersoid particles acting as obstacles to dislocation movement. Comparing the fatigue crack growth behaviour between the high Mn alloys and the commercial 6N01 alloy in the as forged condition, high Mn alloys are shown to have higher fatigue crack growth resistance and show a more tortuous crack path. This result call be explained by the increasing energy absorption through crack deflections and tortuous crack paths by the Mn dispersoids.