The effects of the pre-aging period on the formation behavior of nanoclusters during two-step aging in an Al-3.0Mg-1.0Cu (mass%) alloy has been investigated by the hardness, differential scanning calorimetry (DSC) and electrical resistivity measurement. Two-step aging which consists of pre-aging and final aging results in an increase of the peak hardness. When the pre-aging is conducted at 323 K for 1.2 ks, the hardness dramatically increases after the final aging treatment. This pre-aging period corresponds to the incubation stage which is one of the hardening stages in an Al-Mg-Cu alloy. During the pre-aging, no clear hardness and electrical resistivity increase are obtained, however a certain structural change is considered to occur. In this study, co-clusters are assumed to be formed during pre-aging. On the contrary, when the pre-aging period is 345.6 ks, the hardness and electrical resistivity decrease in the beginning of final aging. It is attributed to the reversion of nanoclusters. In this case, the pre-aging period corresponds to the first stage of hardening. The first stage of hardening, which occurs rapidly (e.g. within 60 s at 443 K) and contributes 60% of the total hardening, is attributed to solute clustering rather than to the formation of GPB zones.