Abstract: Carbon spheres (CSs) were modified by a mixture of HNO₃ and H₂O₂ with different volume ratios and concentrations to improve their dispersability in a liquid. The morphologies and microstructures of the modified CSs were characterized by field emission scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectrometry and acid-base titration. Results show that the amount and types of oxygen-containing groups can be tuned by the concentration and ratio of HNO₃/H₂O₂. HNO₃ favors the formation of carboxyl and carbonyl groups while H₂O₂ introduces mainly hydroxyl. The amount of oxygen-containing functional groups increases with the concentration of the oxidants when the volume ratio of HNO₃/H₂O₂ is 1, which is as high as 2.164 mmol/g when the concentrations of HNO₃ and H₂O₂ are 16 and 8.8 mol/L, respectively. The resulting CSs have a remarkable dispersability and stability in water.