Abstract: Vacuum brazing technology of K644 cobalt-based superalloy was studied with Co-Cr-Ni-W-Al-Ti brazing filler metal. Microstructure and phase composition of brazed joints obtained at different holding times were observed and analyzed by scanning electron microscopy and energy spectrum analysis, and rupture life of brazed joints at high temperature was tested. The results showed that K644 alloy could be well joined with cobalt-base brazing filler metal at 1 220 ℃. Brazing seam was mainly composed of gray Ni-Co solid solution, light gray CrB type skeleton boride phase, white (Cr,Co,W)₃B₂ type skeleton boride phase, white Ti, Ta and W rich irregular and dispersed granular MC type carbide phase. With the extension of holding time, brazing seam became wider, the total amount of compound phase decreased, skeleton became thinner, and composition of compound phase tended to be consistent. The content of γ' phase in the primary crystal of solid solution and between skeleton of compound phase increased obviously, size increased slightly, and shape and distribution became more uniform. Rupture life of brazed joints at high temperature decreased slightly and then increased continuously. The best performance of brazed joints was obtained under the condition of holding time for 120 min, and the average rupture life of brazed joints reached 487 h 21 min.