A simplified equivalent circuit, which is useful in L-band, of switching MOSFET is presented. The MOSFET model accounts for the relatively low resistivity of Si substrate. By using this circuits, the relationship between the MOSFET equivalent circuit parameters and a series-shunt FET connected SPDT switch characteristics has been revealed. In order to evaluate the relationship mentioned above, enhancement type NMOSFETs and a SPDT switch with the FETs are fabricated. The MOSFET equivalent circuit parameters at L-band were extracted from measured small-signal S-parameters of the FETs. The measured switch characteristics are fairly good agreement with the simulated results which has been accomplished by using the MOSFET model. This good agreements shows the effectiveness of the MOSFET model which is presented here.