The single crystal supperalloy DD5 was brazed using Co—Ni—Si—B filler alloy at  1453 K. The microstructure of joint and effect of brazing time on the microstructure and mechanical properties were investigated by SEM, EPMA and TEM. The formation mechanism of joint was discussed deeply. The result indicates that three distinct regions can be identified in a joint: the filler alloy zone, interfacial bonding zone, element diffusion zone. The filler alloy zone in the center of joint consists of Ni—Co solid solution, M₃B₂—type boride, CrB and Ni—Si compounds. The interfacial bonding zone adjacent to the filler alloy zone is composed of Ni—Co solid solution formed isothermally at brazing temperature. The element diffusion zone between the interfacial bonding zone and substrate consists of γ phase, γ′ phase and granular M₃B₂ phase which was formed in element diffusion. With extending brazing time, the brazing defects reduce, the thickness of bonding layer and the size of spherical Ni—Co solid solution in the filler alloy zone increase, and the brittle compounds reduce. The tensile strength at 1143 K increases from 198.5 MPa to 580 MPa. The investigation on fracture of high temperature tensile specimens exhibits that the fracture occurred in the seam and the fractural model is a mixed fracture. Through extending brazing time the bonding strength between seam and substrate can be improved, the brittle compounds can be controlled and the mechanical properties of joint are improved. The analysis results of microstructure and properties also imply that the optimum holding time would be 180 min.