We suggest that a scanning tunneling microscope (STM) may be used for investigating the insulator–conductor interface, in particular SiO2/Si, at nanometer scale. For an insulating film transparent to tunneling, we estimate, using a simple model, the roughness of the interface from the STM image. It is found that the interface roughness is less than the roughness of the image surface times the ratio of effective decay lengths in the film and in vacuum. For relatively wide films, of order 10 nm, STM measurement in the field emission regime can give the interface image with 1 nm precision. ©1995 American Institute of Physics.