A new etching technique for revealing the plastic deformation zone in an Al-Cu-Mg alloy has been developed. The etching with the proposed etching agent was conducted on the deformed sample after being heated to 673 K for 3 h. With this etching technique, the plastic deformation zone was clearly observed even under low magnification. This was due to the change of microstructural characteristics in the plastic deformation zone after the heating process, in which there is significant precipitation of Al2Cu and Mg2Si, caused by the high energy arising from the severe deformation.