Abstract: By using diffusion couple made by inlaying, the diffusion-solution zone of Al/Co was researched under different anneal conditions.The microstructure and forming rule were observed and analyzed by means of SEM and EDS, and its forming mechanism was discussed.The results show that the diffusion-solution zone is formed at Al/Co interphase when heating temperature 600℃ and holding time 75 hours, its thickness is 170 μm, and its structure is Al/Co₂Al₉/Co₄Al₁₃/Co₂Al₅/CoAl/Co, the structure being consistent with sequence of each phase in Al/Co binary alloy phase diagram. CoAl phase layer is firstly formed on Co, and other three layers are then formed on CoAl;the Co₄Al₁₃ phase layer and Co₂Al₅ phase layer grow with the opposite direction in "pole pattern", they turn around landscape orientation to grow from root after they have developed definite thickness, and layer 2 and layer 3 intervene in tow; lastly, Co₂Al₉ phase layer is formed at Al/Co₄Al₁₃ interface.The forming of Al/Co diffusion-solution zone resulted from diffusion, dissolve and crystal of Al and Co under the condition of solid phase. The reciprocity of concentration and solubility caused the formation sequence of diffusion-solution zone.