Abstract: Disclosed herein is a method for chemically attaching carboxyl, alcohol, amine or amide groups to the surface of nanodiamond (ND) in a liquid phase. Also disclosed herein are a functional ND compound obtained by the method and use thereof. The method includes treating synthetic ND with a size of 1 nm-1OO nm with sonication and a strong acid to provide ND-(COOH)n. The ND-(COOH)n compound is used as a starting material to provide ND compounds having alcohol, amine or amide groups attached to the surfaces thereof. The surface-functionalized ND compounds are characterized by using an X-ray diffractometer, FTIR, AFM, particle size analyzer and zeta sizer. The ND compounds show functionalities as well as high solubility to provide stable ND solutions in a liquid phase. Therefore, the ND compounds may be used as diamond coating agents. The powder of the ND compounds may be used as materials for producing composites of polymers, plastics, synthetic fibers, ceramics, etc.