Inresponse to the urgent demand for highly reliable joining of heterogeneous nickel-based superalloys in aerospace applications, transient liquid phase diffusion welding (TLP) of dissimilar nickel-based superalloys GH4099 and GH3128 was studied. The influence of the bonding temperature on the microstructure and mechanical properties of the joints was systematically investigated, providing theoretical guidance for welding dissimilar materials. BNi-2 was used as an interlayer, and the joint microstructures were characterized via EBSD, EPMA, and SEM. Tensile tests were performed to evaluate the mechanical properties. The results indicate that with increasing bonding temperature, grain growth gradually occurs in the isothermal solidification zone (ISZ) and that the borides along the grain boundaries in the diffusion-affected zone (DAZ) change from a continuous to a discontinuous morphology. The joint bonded at 1 120 ℃ exhibits the best mechanical performance, with an ultimate tensile strength of 953 MPa, equivalent to 117.1% of the GH3128 base metal.