Abstract: The present application provides a method for monitoring a gate oxide thickness: providing a device structure comprising a gate structure, a gate oxide layer under the gate structure, source and drain regions and a base region; applying a voltage ?Vdd on the gate structure so that an accumulation layer is formed between the source and drain regions, applying a small AC voltage on the basis of the gate voltage ?Vdd; grounding the source and drain regions; applying a voltage signal close to 0 potential on the base region; obtaining the capacitance Cox between the gate structure and the base region by testing; and obtaining the thickness of a gate oxide layer according to the formula Tox=?*S/Cox. This technique accurately monitors the thickness of the gate oxide layer, and avoids those errors caused by existing methods.