The radical addition to the smallest fullerene C-20 has been investigated by means of density functional theory (DFT) method in order to elucidate the radical scavenge mechanism of fullerene. The OH radical was examined as an organic radical because the radical has a high reactivity. The DFT calculation showed that the OH radical binds directly to the carbon atom of C-20 and a strong C-O bond is formed. The binding energies of the first addition of OH radical were calculated to be 85.2 kcal/mol at the B3LYP/6-311G(d,p) level of theory. In the second radical addition, the binding energy of OH to C-20(OH) was 91.5 kcal/mol. The unpaired electron was distributed widely over the C-20 surface in the C-20(OH) complex.