Through microstructure observation and X-ray diffraction analysis, the equilibrium phase constituents of Al-Zn alloythat contains 2 at. pct Cu at room temperature have been determined as Al-based solid solution (α), Zn-based solidsolution and Al4Cu3Zn phase (T′-phase), which are different from α phase, Zn phase and CuZn4 phase originallybelieved. It is determined that the products of discontinuous precipitation transformation below 277℃ are not theequilibrium phase constituents, but the metastable phases made up of α phase, Zn phase and CuZn4 phase. Thephase constituents after discontinuous precipitation of AlZn-2Cu alloy would transform to the ones in equilibriumstatus: Al-based solid solution (c) in fcc structure, Zn-based solid solution in hcp structure and Al4Cu3Zn phase(T′-phase) ultimately through plastic deformation at room temperature and re-heating treatment below 277℃.