The mechanical properties of Zn-22% Al alloys were measured with the low-frequency forced vibration mode. It was observed that a typical transformation internal friction peak was associated with eutectoid decomposition. A relaxation peak in the complex elastic modulus M2-T curves with an activation energy of 100±1.2kJ/mol was observed at the low-temperature side of the eutectoid decomposition peak. It was deduced that the relaxation process was related to the diffusion process of Al atom along the α/β phase boundary and α/α crystal boundary.