In this work, the isothermal solidification stage during transient liquid-phase bonding (TLP) single-crystal superalloys has been investigated. Experiments were performed to ascertain the bonding microstructures and the kinetics during the isothermal solidification. The results have shown that the isothermal solidification stage deviates from the standard parabolic TLP models. Lots of the borides with fine, short bar and acicular morphologies formed in the diffusion affected zone (DAZ) in the thick wall and thin wall substrate specimens at the isothermal solidification stage. Electron probe microanalysis results have shown that there exists B composition peak in the DAZ. Examination of the bonding kinetics presented that there are three stages in the isothermal solidification stage: initial stage, transient stage and final stage with different growth velocity of the isothermal solidification zone (ISZ). And the relationship of the width of the ISZ with the square root of the bonding time didn’t satisfy the parabolic relationship. Based on the microstructures and kinetics observed, a film ISZ mechanism is proposed, and a model is constructed to illustrate the isothermal solidification stage during bonding single-crystal superalloys.