A method for measuring the thickness of a very thin oxide layer by secondary electron emission is described. The secondary electron signals are measured for an oxide‐covered silicon surface and a reference surface by using the line mode of scanning electron microscope. Then, their ratio correlates well with the thickness of oxide layer, and it is demonstrated that the thickness of an oxide layer can be obtained by measuring the ratio &ggr;. At the beam energy of 2 keV, the measurable thickness lies within the range of 15–150 A˚. The lower limit is due to the natural oxide remaining on the reference‐silicon surface and the upper limit to charging effects of oxide layers. A remarkable feature of this method is that an oxide thickness localized in a very small area is measurable, and the measurement is carried out for an oxide deposided in 0.1‐mm‐wide ditches.