Abstract: By using diffusion couple made by inlaying,the diffusion layer of Al/Mg was researched under different anneal condition.The microstructure and forming rule were observed and analyzed by means of scanning electron microscopy(SEM) and Energy Disperse Spectroscopy(EDS),and its forming mechanism was discussed.The results showed that the diffusion layer was formed at Al/Mg interphase with heating temperature of 580 ℃and holding time of 60 hours,its thickness was 265 μm,its structure was Al/ Mg₂Al₃/ MgAl/Mg₃Al₂/Mg,which included all phases in Al/Mg binary alloy phase diagram,and the structure was consistent with the sequence of each phase located Al/Mg phase diagram.The solid phase diffusion,dissolve and crystal of Al and Mg resulted in forming of diffusion layer with different diffusion temperature and time,and the diffusion layer was almost formed at Al/Mg interphace at the same time,which was different number of layer,thickness and structure.