SOI MESFETs fabricated using a commercial SOI CMOS process  demonstrate excellent RF capabilities and can operate at voltages that are at least 3X higher than the MOSFET breakdown voltage. These features make the SOI MESFETs an attractive RF power amplifiers candidate. However, a MESFET-based PA design for RF integrated circuit applications requires an accurate largesignal device model. We propose a large-signal modeling strategy for the SOI MESFETS using widely available GaAs based MESFET models for initial model development. A measurement-based approach is then used to adapt the initial model to a TOM3 model  to represent the SOI MESFETs. We have extracted a small signal model using s-parameters measured up to 8 GHz under different bias conditions and developed a charge-based large signal model for the device. The models can be used in commercial, large-signal circuit simulators.