Gallium phosphide (GaP) nanowire transistors were fabricated in a back gated structure and their electrical characteristics were measured systematically in air and vacuum. Typically the transistors turn on at -7 to -5V in ambient air. However, in vacuum, a large Vth shift about ∼10V toward the negative gate bias was observed. Exposure to air for 48 hours shifts the Vth back to the original Vth in air, which implies a reversible Vth shift. We believe that the shift of Vth is associated with charge transfer between physically adsorbed O or OH species and the surface of GaP nanowire. The device operation can be explained by conventional n-channel depletion type MOSFET operation.