Recently, extensive studies have been conducted [1-5] in order to realize enhancement-mode III-V MOSFETs by improving the interface between gate oxide and III-V channel. The carrier mobility in advanced substrates is generally regarded as a figure of merit in benchmarking high-mobility channel materials against bulk Si substrates. However, charge trapping can be a source of error in mobility calculation using split C-V method for Si MOSFETs with high-k dielectrics . On the other hand, magnetotransport measurements are suitable for direct measurement of inversion charge density (Ninv) and mobility in MOS devices with high-k gate dielectrics, where significant charge trapping makes evaluation of inversion charge density using split C-V method inaccurate. In this work, we employ gated-Hall-bar (GHB) structures to directly measure the inversion charge and mobility in a GaAs MOSFET.