GaAs power MESFET structures have been grown by MBE on Si substrates. Growth specifications are the nucleation of GaAs on Si by migration enhanced epitaxy and the annealing of the first few hundred angstroms-thick GaAs layer before using standard MBE growth conditions for GaAs. A standard process with an air bridge source interconnection and 0.8 µm gate length has been achieved. 1 W output power with a 1 dB compressed gain of 5 dB and 1.6 W with a 1 dB compressed gain of 9 dB are obtained at 12 and 5 GHz, respectively. The power and the gain are equivalent to the same GaAs/GaAs MESFETs. The drain efficiency is 35%, which is excellent.