alpha(6H)- and beta(3C)-SiC powders were sintered with the addition of AlB2 and carbon. alpha-SiC powder could be densified to similar to98% of the theoretical density over a wide range of temperatures from 1900degrees to 2150degreesC and with the additives of 0.67-2.7 mass% of AIB(2) and 2.0 mass% of carbon. Sintering of the beta-SiC powder required a temperature of >2000degreesC for densification with these additives. Grains in the alpha-SiC specimens grew gradually from spherical-shaped to plate-shaped grains at 2000degreesC; the 6H polytype transformed mainly to 4H. On the other hand, grains in the beta-SiC largely grew at >2000degreesC; the 3C polytype transformed to 4H, 6H, and 15R. The stacking faults introduced in grains were denser in beta-SiC than in alpha-SiC. The rapid grain growth in the beta-SiC specimen was attributed to polytype transformation from the unstable 3C polytype at the sintering temperature.