Abstract: Through microstructure observation and X-ray diffraction analysis, characteristic of transformation mechanics of discontinuous precipitation in Al-Zn alloy with symmetrical composition under 277°C were investigated after attaining its single phase microstructure by solution treatment. It is found that spinodal decomposition and subsequent continuous coarsening occur during the incubation period. The discontinuous precipitation shows lamellar microstructure composed of Al-based solid solution (a) with f. c. c. structure and Zn-based solid solution (β) with h. c. p. structure. The nucleation site depends on the transformation temperature and the cardinal nucleation sites take place along the grain boundaries. The transformation dynamics follows the relationship of Avrami Equation, whose exponent is 2.