Crystallite sizes of various carbon blacks were evaluated by Raman spectroscopy and X-ray diffraction. Carbon blacks were experimentally produced by the pyrolysis of ethylene or acetylene. The experimental carbon blacks, commercial carbon blacks (#15, AX-015, #60, #51 and #70, ASAHI CARBON CO., LTD.), and the same commercial carbon blacks heated at different temperatures, 1473, 1673, 1873 and 2473 K were examined. The D1/G ratio (i.e. R ratio) and crystallite size La were determined from the Raman data and X-ray diffraction patterns, respectively. The R ratio decreased with increasing La in the case of La>2 nm. This trend is consistent with a previous study in which the R ratio was proportional to 1/La, while the proportionality constant was different from the previous study. In the case of La<2 nm, the R ratio increased with increasing crystallite size, and the R ratios of the experimental carbon blacks were larger than those of the commercial ones, whereas the R ratios of the heated samples of these experimental carbon blacks were similar to those of the commercial ones. This result is attributed to volatile matter at the particle surface. These results indicate that the R ratio has a maximum value at La=2 nm, and it is affected by the surface characteristics of the particles.