Abstract: Microstructure of transient liquid phase (TLP) diffusion bonded N5 single crystal superalloy joint had been investigated using scanning electron microscopy (SEM), backscattered electron (BES) and electron back scattering diffraction (EBSD) for understanding mechanism of single crystallization of joint. The results showed that formation of transjoint grain boundary and equiaxed grain in joint resulted in non single-crystallization joint. During bonding, those should be controlled carefully for machining of specimens, accession of interlayer, fitting, bonding and so on. During isothermal solidification process, liquid commenced to solidification by the flat crystal growing epitaxially from the unmelt single crystal base metal until liquid disappeared. Common single crystal grain with uniform crystal orientation at the interface was formed. Sufficient homogenization process was carried out with increasing welding time. After cooling of joint, single crystallization of joint would be obtained.