TC4-LY12 welding joint are fabricated by vacuum diffusion welding. The bonding phases are confirmed by X-Ray Diffraction (XRD), the morphology of samples and chemical composition are evaluated by Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray Spectrometer (EDS) analysis, the mechanical property is tested by tensile strength measurement. The XRD patterns show that the main intermetallic compounds formed in the transition zone are Ti3Al, TiAl and TiAl3. Combining the fracture surfaces morphology and element analysis, the bonding mechanism of TC4-LY12 joint is attributed to the inter-diffusion between Ti and Al atoms. The initial phase formed during the reaction is TiAl3 due to the diffusion of titanium atoms into aluminum substrate; with the process of the reaction, Ti3Al and TiAl phases appear gradually with the atomic ratio of 1:1.The tensile strength of the joint increases remarkably with increase of temperature and takes a maximum value (~60MPa) when the welding temperature is 550°C.