Characteristics of the dielectric breakdown and the thickness homogeneity of silicon gate oxides have been investigated on the nanometre scale using conducting atomic force microscopy (C-AFM). The I -V measurements in the Fowler-Nordheim tunnelling regime were applied in order to use C-AFM as an experimental technique for the thickness determination of thin silicon gate oxides. Conducting AFM was used further to study dielectric breakdown with a lateral resolution below 100 nm. It was found that the accumulation of defects created by induced charges and a statistic path formation of these defects are the main mechanisms leading to dielectric breakdown.