The effective electron mobility ofN‐channel MOSFET’s with different gate oxide thickness has been measured at 4.2 K as a function of electron density. An increase in the peak effective mobility is observed as the oxide thickness is increased. It is found that the peak mobility is determined by the scattering mechanism whose scattering probability is independent of electron density. To describe these experimental results, the scattering by a short‐ranged potential has been assumed and discussed.