This letter discusses the abnormal capacitance-voltage (C-V) curves found in MOS transistors with ultra-thin gate oxide less than 2.0 nm. For MOSFETs without source and drain connected to ground, the measured C-V curves in strong inversion state are always high frequency style. In contrast, for MOSFETs with source and drain connected to ground, the measured C-V curves in strong inversion state indicates low frequency style. Models to interpret these abnormal C-V curves are proposed and discussed in detail. (C) 2002 Elsevier Science Ltd. All rights reserved.