Microstructure of transient liquid phase( TLP) diffusion bonded a third generation single crystal superalloy joint was investigated using scanning electron microscopy( SEM),and mechanical properties test of joint was carried out,for obtaining relationship between microstructure and mechanical properties of joint. The results showed that the joint contained bonding zone and base metal. The diffusion zone was obviously observed. When it was not finished for isothermal solidification process,the bonding zone would contain isothermal solidification zone and rapid solidification zone. Metallographic examination revealed that isothermal solidification zone was consisted of γ and γ’ phase. Rapid solidification zone was consisted of two different structures,which were ternary eutectic of borides,γ and γ’ phase developing at the edge of joint,binary eutectic of γ and γ’ phase appearing in the portion of joint. When it was not enough for homogenization process under the condition of finishing isothermal solidification process,the bonding zone would contain isothermal solidification zone and borides at the interface. Under the conditions of relatively high welding temperature and long welding time,average tensile strength of joint was equivalent to that of parent material. Microstructure of transient liquid phase (TLP) diffusion bonded a third generation single crystal superalloy joint was investigated using scanning electron microscopy (SEM), and mechanical properties test of joint was carried out, for obtaining relationship between microstructure and mechanical properties of joint. showed that the joint contained bonding zone and base metal. The diffusion zone was obviously observed. When it was not finished for isothermal solidification zone, the bonding zone would contain isothermal solidification zone and rapid solidification zone. of γ and γ ’phase. Rapid solidification zone was consisted of two different structures, which were ternary eutectic of borides, γ and γ’ phase developing at the edge of joint, binary eutectic of γ and γ ’phase appearing in the portion of joint When it was not enough for homogenization process under the condition of finish ing isothermal solidification process, the bonding zone would contain isothermal solidification zone and borides at the interface. under the conditions of relatively high welding temperature and long welding time, average tensile strength of joint was equivalent to that of parent material.