Scanning tunneling microscopy for the observation of the surface topography and quartz crystal microbalance for the micro mass measurement of the thin film have been combined to investigate the morphology change of the surface of Ag thin film. Ag is evaporated on the surface of the AT-cut quartz crystal which is oscillated at the resonance frequency. The mass change for the adsorption or the desorption results in a shift in the resonance frequency. Thus the mass of the adsorption or the desorption can be measured by detecting the frequency shift of the quartz crystal oscillator. In situ morphology and mass changes as little as 320 ng cm(-2) in the NH3 adsorbed process on Ag surface are observed by the combined system of the scanning tunneling microscopy and the quartz crystal microbalance.