The effects of the thickness of the GaAs strained buffer layer on the electrical performance of GaAs field effect transistors are demonstrated. The GaAs MESFET has been fabricated on GaAs/InP epitaxial material grown by molecular beam epitaxy. From experimental results a minimum thickness of the strained buffer layer appears to be needed to compensate the effect of misfit dislocation due to lattice mismatch between the GaAs and InP substrate. The minimum required thickness amounts to 2 mu. The electron mobility and transconductance behavior demonstrate the effects of thickness of the strained buffer layer.