Dense Cf/(ZrHfNbTaCr)B2–SiC (CBS‐Co) composite is successfully prepared at a low temperature of 1500°C by adding 5 vol.% Co. Liquid Co accelerates the particle rearrangement and significantly enhances the relative density of CBS‐Co (95.2%). In situ formed Co2B phase by diffusion reaction bonds tightly with high entropy diboride (HEB) and SiC grains in CBS‐Co. Due to the promotion of relative density and interfacial bonding strength, CBS‐Co exhibits a high flexural strength (283 ± 23 MPa). Structural damages of carbon fibers are effectively prevented by compact fiber coating. The well‐preserved fibers play dominant roles to increase the fracture toughness of CBS‐Co (4.77 ± .5 MPa·m1/2). Moreover, SiO2‐rich oxide layer can effectively heal flaws and inhibit oxygen diffusion, achieving relatively high flexural strength after oxidation at 1300°C of CBS‐Co composite. This work provides a stepping stone for developing high‐performance carbon fiber toughened HEB composites based on low temperature liquid‐phase sintering technology.