Abstract: Ni-based single crystal superalloy DD5 was brazed by using Co-based braze alloy at 1180℃ for 60 min. The microstructure of the joint and effect of the thickness of brazing seam on the microstructure and distribution of phases were investigated by SEM and EPMA in this research. The result indicates that the brazing seam was composed of γ+γ' and dispersive M₃B₂ precipitates when the thickness of seam is 10 μm, which is similar to substrates. With the thickness of brazing gap increasing, several kinds of compound were formed in the seam, which is the skeleton shaped M₃B₂ phase, γ-Ni solid solution rich Co, Ni-Si compounds and other brittle phase. When the brazing gap is over 200 μm, the spherical Co-based solid solution is formed in the brazing seam. Several kinds of compound are formed in the interface of Co-based solid solution. The testing results of mechanical properties show that the high temperature tensile of the joint improves with the decrease of brazing gap. The tensile of joint at 870℃ can reach the highest value of 792 MPa.