Prednisolone (PSL), an anti-inflammatory glucocorticoid drug, was adsorbed on oxidized single-walled carbon nanohorns (oxSWNHs) in ethanol-water solvent. The quantity of adsorbed PSL on the oxSWNHs was 0.35-0.54 g/g depending on the sizes and numbers of holes on the oxSWNHs. PSL was adsorbed on both the outside and the inside of the oxSWNHs, and released quickly in a couple of hours and slowly within about one day from the respective places. The released quantity in culture medium strongly depended on the concentration of the PSL-oxSWNH complexes, suggesting that PSL adsorbing on oxSWNHs and PSL in the culture medium were in concentration equilibrium. The local injection of PSL-oxSWNHs into the tarsal joint of rats with collagen-induced arthritis (CIA) slightly retarded the progression of the arthritis compared with controls. By histological analysis of the ankle joint, the anti-inflammatory effect of PSL-oxSWNHs was also observed.