The spatial scaling law of the Si/SiO2interface roughness was investigated with atomic force microscopy. Scaling behavior was observed on smaller scales, where the root‐mean‐square (RMS) roughness increased as a power of the scale of observation. When viewed as a fractal geometry, such a structure is characterized as aself‐affinefractal. On larger scales, the roughness was no more dependent on the scale, showing the (macroscopic) RMS roughness in the conventional sense. The observed structure (self‐affine fractal with a finite‐length cut‐off) is consistent with the prediction of the theory of kinetic roughening in a far‐from‐equilibrium growth, where the fluctuation on smaller scales evolves into roughness on larger scales. Statistical description of the Si/SiO2interface roughness was also given in terms of autocorrelation function and power spectral density. It was found that the autocorrelation function of the Si/SiO2interface roughness is well approximated by an exponential form rather than a Gaussian form. Numerical simulation was carried out to explain the kinetic roughening in the oxidation process with relation to the dependence on the oxide thickness.