The structure of mesophase carbon powders prepared by heating coal-tar pitch at different temperatures was studied using the small-angle X-ray scattering method. The wide-angle diffraction profiles of mesophase carbon powders indicate only the first peak at q = 18 nm-1 and subsequent broad oscillations. A single graphite layer is suggested to be one of the local structural orderings in the present mesophase carbon powders similar to those of carbon black and amorphous carbon. The small-angle X-ray scattering data clearly suggest that mesophase carbon powders include structural inhomogeneity, and size is relatively small on the order of 10 nm in gyration radius. A fraction of small size inhomogeneity increases according to change in heating temperature (i.e., C/H ratio). The Porod plot of the scattering intensity also suggests that some significant structural change is induced by graphitization in mesophase carbon powders at the early stage of heating.