Abstract: Microstructure of transient liquid phase (TLP) diffusion bonded single crystal superalloy joint was investigated HJ1.2mmby 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 was contained by the bonding zone and parent material zone. Before isothermal solidification process was finished, the bonding zone was contained isothermal solidification zone and rapid solidification zone. Metallographic examination revealed that isothermal solidification zone consisted of γ and γ' phases, rapid solidification zone consisted of eutectics. When homogenization process was not enough under the condition of finishing isothermal solidification process, the bonding zone would contain isothermal solidification zone and borides at the interface. It is effectively improved for mechanical properties of joint by welding procedure of "isothermal solidification at relatively low welding temperature and homogenization at relatively high welding temperature".