Threshold voltages of Si-implanted gallium arsenide (GaAs) MESFETs depend on not only wafer crystallographic characteristics, but also MESFET fabrication processes. When the position of the orientation flat was changed to improve mechanical shock tolerance, it was found that Vth varied on the wafers due to a crystallographic orientation effect on the Si implantation. Thus, we investigated Vth distributions on wafers of several different Si implantation orientations. It was confirmed that Vth largely varies according to Si-implantation orientations.