Abstract: With the development of channel down-scaling of low-voltage low-power CMOS technology, the optimal operation points are shown to shift from the strong inversion toward lower moderated inversion and weak inversion regimes. High-frequency equivalent circuit modeling is a prerequisite for finding the physical mechanism of MOSFET device, and is essential for the HF integrated circuits. Based on the physical structure of 45 nm MOSFET device and its Y parameters analysis, a quasi-static approximate RF equivalent circuit model and its high-precision simplified parameter extraction algorithm is proposed by taking into account the intrinsic physical characteristics of the device, the electromagnetic characteristics of the pin and the parasitic characteristics of the test pad and test interconnects, which are used to describe the bias dependence from the strong inversion and weak inversion regimes. Therefore, the device characterizations offer excellent accuracy, continuity and smoothness under different bias condition, and can be easily implanted into commercial EDA tools. Direct extraction method is performed by S-parameter analysis including the intrinsic and the extrinsic components and the substrate-related effect. Finally, the practicability and the accuracy of the proposed model and its parameters extract algorithm are verified by the consistency comparison of the simulated results by using ADS2013 tool and the measured S parameters. The experimental results show the bias dependence of 45nm MOSFET.