The dispersion state and aggregate structure of carbon black (CB) in polystyrene (PS) composites prepared by solvent casting the suspensions in different dispersion media: tetrahyrdrofuran, chloroform, and toluene, were investigated by transmission electron microscopy, and ultrasmall-angle and small-angle X-ray scattering (USAXS and SAXS). The macroscopic dispersion state of CB in the solvent-cast films reflects the dispersion stability of the suspensions in the PS solutions with the different dispersion media. The mass and surface fractal dimensions, D_m and D_s, of CB in PS were evaluated by applying the Beaucage unified equation to combined profiles of USAXS and SAXS. The evaluated values of D_m tend to exhibit a increment for the composite prepared by dispersion media with lower CB dispersibility in suspension, while the D_s values are clearly lower than that for the CB powder due to the effects of physical adsorption of the polymer on the CB surface. The viscoelastic moduli of the melt composites below and above the percolation limit varied with the dispersion medium, reflecting the difference in the macroscopic dispersion state and aggregate structure of CB in PS.