α(6H)-SiC powders mixed with 11 types of metal boride, B and C were sintered at high temperatures (1800-2200°C for 30min). MoB, NbB2, TaB2, TiB2, VB2, WB, and ZrB2 promoted the densification of SiC powder and sintered SiC had a high fracture toughness (4.07-4.75MPa·m1/2). The addition of metal boride enabled us to control the grain growth and densify SiC powder containing a fairly large amount of B. Metal borides dispersed in SiC grains, precipitated in triple points, and appeared to obstruct grain growth and crack propagation. CaB6, CrB, LaB6, and YB6 did not promote sintering of SiC powder. These latter metal borides resulted in highly exaggerated grain growth and the SiC powder became porous after sintering. The starting 6H-SiC powder tended to be partially transformed into a 4H polytype which was accompanied by grain growth.