Precise simulation of non-quasi-static (NQS) characteristics is crucial for the analog application of MOS transistors. This paper presents the small signal admittance model of four-terminal NQS MOS transistors by solving the differential equation derived from the primary principle. The model contains the bulk-charge effect, the mobility reduction, and the velocity saturation. The results are compared with those for the conventional quasi-static model, the BSIM3v3 NQS model, and the 2-D device simulation.