Abstract: Using Ni foils of different thicknesses (10, 50, and 100μm), contact reaction brazing of Ti65 and Nb521 alloys was performed at 1150°C for 20 min. The influence of Ni foil thickness on interfacial microstructure evolution and mechanical properties was investigated. Results show that Ni foil thickness critically governs interfacial reactions: increasing foil thickness leads to a progressive increase in reaction layer thickness. At 100μm, interfacial reactions are markedly intensified, producing a thick, complex reaction layer with abundant brittle intermetallic compounds. Although all joints exhibit dense metallurgical bonding, their mechanical properties differ significantly. The joint brazed with a 50μm Ni foil achieves the highest shear strength of 394MPa, with fracture occurring in the Nb521 base metal. In contrast, excessive brittle phase formation with a 100μm Ni foil degrades interfacial bonding and significantly reduces shear strength. These results demonstrate that controlling Ni foil thickness is an effective approach to tailoring the interfacial microstructure and mechanical performance of Ti65/Nb521 dissimilar metal brazed joints for high-temperature applications.