Accurate modeling and efficient parameter extraction of a small signal equivalent circuit of MOS transistors for high-frequency operation are presented. The small-signal equivalent circuit is based on the quasi-static approximation which was found to be adequate up to 10 GHz for MOS transistors fabricated by a 20 GHz cutoff frequency technology. The extrinsic components and substrate coupling effects are properly included. Direct extraction is performed by Y-parameter analysis on the equivalent circuit in the linear and saturation regions of operation. A low-noise amplifier is used to illustrate the effects on circuit performance due to accurate inclusion of extrinsic components in the model. Good agreement between simulated results and measured data on high-frequency transistor characteristics has been achieved.