The effects of the aging conditions on the serrated flow in Al–Mg (–Zn) alloys were investigated, focusing on the precipitation states. Al–6%Mg and Al–6%Mg–3%Zn alloys were naturally and artificially aged after solution treated and quenched. The serrated flow was examined by tensile test. As for the Al–6%Mg–3%Zn alloy, the naturally aged specimens show the increase in the critical strain for serrated flow. The Zn–Mg clusters are formed during natural aging. Furthermore, the increase in the number density and the size of these clusters corresponds to the increase in the critical strain, which reveals that the formation of these coherent clusters is one of the dominant factors for delaying the onset of serrated flow. On the other hand, as for the artificially aged specimens in the under-aged conditions, the transformation of the clusters to the incoherent meta-stable precipitates brings about the decrease in the critical strain. Furthermore, the increase in the volume fraction of these precipitates in the over-aged conditions reduce the stress amplitude of serrated flow, which is brought about by the decrease in the solute Mg content.