Al-Mg-Cu alloys are known to rapidly harden after a short period of artificial aging. The cause of the hardening is still disputed in the literature. This work addresses the rapid hardening issue by High Resolution Transmission Electron Microscopy (HRTEM) studies coupled with image processing and image simulation. HRTEM observations made on an Al-3Mg-0.4Cu alloy reveal the presence of ordered zones after 1 hour aging at 180degreesC with the additional appearance of the S" phase formation after 4 hours aging. These findings suggest that it is the presence of these ordered zones of Mg atoms in the aluminum matrix which leads to the initial stage of hardening in this and similar alloys with small Cu:Mg ratio.