Transient liquid phase (TLP) bonding was investigated in Hastelloy-X samples with different filler metal thicknesses (20, 35, 50, 65, and 100 μm) and holding time (5, 20, 80, 320, and 640 min) to obtain optimum bonding parameters. Microstructural evaluations using electron probe microanalysis (EPMA) and electron backscattered diffraction (EBSD) show that the central eutectic phases present in the athermally solidified zone (ASZ) are Ni₃B, Ni₂Si, and CrB, and the precipitates formed in the diffusion-affected zone (DAZ) are MoB, CrB₂, and Mo₂B₅. According to the results, decreasing the filler thickness as well as increasing the holding time helps realize the completion of isothermal solidification and reduction in the density of precipitates in the DAZ, leading to a joint with more uniform properties. Diffusion of boron and silicon to longer distances with increasing holding time causes the removal of Cr-rich borides in the DAZ and the formation of Mo-rich silicide at the joint interface. Decrease in hardness of ASZ and DAZ due to the elimination of brittle phases in these zones during long holding time causes more uniform hardness distribution in the joint area. The best results are obtained for the sample joined with the 35 μm-thick filler metal for 640 min holding time.