Surface-functionalized TiO2 (anatase) nanocrystals (NCs) (avg. d=6.7±1.6nm) were synthesized by a supercritical hydrothermal method. A phosphonic acid derivative was used as surface modifier of TiO2 NCs. It was verified that the phosphonic acids were bound to the surface of TiO 2 with very strong binding energy. The TiO2 NCs also exhibited extraordinarily high chemical stability. Such a strong binding between the phosphonic acid modifier and TiO2 had not been obtained by any other methods. The TiO2 NC showed sharp pH sensitive dispersion. Because of the functional as well as stable surface modifier layer, further manipulations on the surface using chemical reagents could be possible.