Al/Ti diffusion couples were made by embedded technology and diffusion heat treatment was carried out between the melting point of Al and that of Ti. The microstructure evolution, formation mechanism and growth law of Al/Ti diffusion solution zone were investigated. The results show that the microstructure of the diffusion solution zone after heat treatment is the mixture of TiAl3 and Al solid solution with a few Ti; TiAl3 forms first and is the only new phase during heat treatment. The growth mechanism and direction of the diffusion solution zone change with prolonging of treating time; first, its growth is controlled by reaction regime and the growth direction is to Ti substrate, and the growth rate and holding time have linearity relation; afterwards, its growth mechanism changes to diffusion regime and the growth direction is to Al substrate, and the growth rate and holding time have parabola relation. For certain time, the thickness of diffusion solution zone and the treating temperature have the index relation.