The precipitation behavior of a high purity Fe-1.03 wt. %Cu alloy during ageing at 650 C has been investigated bytransmission electron microscopy. The microstructure andthe corresponding diffraction patterns of the {100}*,{110}* and {111}* reciprocal planes were analyzed. A metastablephase with a B2-like structure was observed, whichis quite different from the equilibrium copper phase shownin the Fe -Cu phase diagram. The formation mechanism ofthe B2-like structure is deduced based on the energeticpoint of view.