We report on the novel design of the near-field scanning optical microscope (NSOM) which operates in liquid environment. A resonance tracking digital scanning method is applied to compensate the resonance shift due to the evaporation of the liquid in the atmospheric pressure. By this method, stable operation of NSOM system is demonstrated by showing topographic images of the metallic grating embedded in liquid environment. (C) 2013 Elsevier B.V. All rights reserved.