Structure and forming process of the Ti/Al diffusion bonding joints were studied by means of scanning electron microscopy (SEM), X-ray diffractometry (XRD) and shear strength measurement. The TA2 and L4 were used as bonding couples. The results showed that the forming process of joints included four stages. The primary and unique phase was TiAl3 from the diffusion reaction for long time. There was a delay time tD before TiAl3 appeared, mainly affected by temperature. The joint strength depended on the metallurgical combination percentage and interface structure of diffusion zone, reaching or even exceeding the strength of L4 when the TiAl3 formed like sheet and layer. The shearing fracture of joints occurred on the interface of Al and diffusion zone or in the Al-based metal.