Abstract: Al/Ti diffusion couple made by inlaying was annealed under and above Al melting point, respectively, and solid-solid and solid-liquid diffusion couples were formed. the microstructure of Al/Ti solid-solid and solid-liquid diffusion layers were observed by OM and SEM-EDS, and the formation mechanism was investigated. The results show that Al/Ti solid-solid diffusion layer was composed by one layer of TiAl₃ But the solid-liquid diffusion layer was composed of single phase layer of TiAl₃ and dual-phase layer of TiAl₃+Al(Ti) solid solution. The formation of dual-phase layer changed regularly from Al to Ti. The Al/Ti solid-solid TiAl₃ diffusion layer and solid-liquid single-phase TiAl₃ layer were formed from diffusion of Al into Ti by forming AI-Ti solid solution with Ti as solvent. However, dual-phase layer TiAl₃+Al(Ti) solid solution formed by dissolving and spread of Ti to the liquid Al by forming Al-Ti liquid solution. It resulted in morphological variation of dual-phase layer TiAl₃+Al(Ti) solid solution that Ti atom concentration gradually increased in Al solution from Al to Ti.