The phase constitutes and phase compositions in the eight alloys designed with dif-ferent compositions of Al--Zn--Cu system have been determined after the homogenoustreatment and then equilibrium cooling to 20℃ by use of optical microscope, electronprobe microanalysis and X--ray diffraction. It has been found that there existed the T’phase in the seven alloys. Consequently, it was testified that the T’ phase was stableat room temperature. At the same time, the phase relationship was not locally rightfor the isothermal section of 20℃ of Al--Zn--Cu system of the ASM published in 1997. The phase constituted and the compositions in the eight alloys designed with dif-ferent compositions of Al-Zn-Cu system have been determined after the homogenoustreatment and then equilibrium cooling to 20 ° C by use of optical microscope, electronprobe microanalysis and X- It has been found that there existed existed the not existed right for the isothermal section of the T’phase in the seven alloys. 20 ° C of Al - Zn - Cu system of the ASM published in 1997.