Abstract: The CMSX-4 single crystal superalloy was brazed by a Ni-based braze alloy containing melting point depressant elements B and Si, and the microstructure and element distribution of the joint were analyzed by SEM and EPMA. The diffusion mechanism of the melting point depressant elements B and Si and the forming mechanism of joint were investigated. The results indicated the joints with different brazing gap demonstrated similar microstructure and phase composition. However, as the width of the brazing gap increased, the precipitation of boride in the seam increased, while defects such as pores began to appear simultaneously. As the holding time increased, the average size of boride slightly increased and its distribution is more concentrated, and the thickness of the interface bonding zone between the base material and the brazing seam increased. During the brazing process, B mainly concentrated on the central of the seam and reacted with elements such as Co, W, Mo, etc. to form a brittle boride phase M₃B₂. Note that no brittle phase precipitated in the near-seam base metal. It can be inferred that B does not diffuse into the base metal. The Si element not only formed a silicide phase in the central of the seam but also diffused into the matrix material, thereby forming a Si-containing solid solution phase in the near the brazing seam. The stress rupture properties of joints with different gap and holding time were tested at 980 °C/100 MPa. It was found that the stress rupture life of the joint decreased with the increase of the weld gap, and increased with the extension of the holding time. When the holding time further prolonged, the stress rupture life was not significantly increased.