The quantitative determination of SiC polytype distributions is very difficult using traditional X-ray powder diffraction quantitative analysis methods, because the diffraction patterns of the various polytypes partially superimpose. The whole pattern fitting technique encapsulated in the Rietveld method is introduced to solve this problem. The detection limits for each polytype can be estimated based on their standard deviations. The results of both synthesized and experimental diffraction data show that the Rietveld method can give precise and accurate percentage compositions of the four most common SiC polytypes. This approach provides a practical tool to relate the preparation conditions to performance properties of SiC-based materials.