To clarify the influence of impurities on the sintering of SiC powder, three 6H-SiC powder samples-with different levels of SiO2 and aluminum impurities-were sintered with additions of boron and carbon. The densification, grain growth, and transformation of 6H-SiC during sintering were studied qualitatively. The powder that contained the most SiO2 required the greatest amount of boron additive for complete densification, SiO2 apparently reacted with the boron additive and was consumed during sintering. The powder with the greater aluminum impurity level exhibited partial transformation of 6H-SiC to 4H-SiC, and the sintered SiC from this powder had elongated grains. The partial transformation in the SiC crystal accelerated non-equiaxial grain growth.