The off-state shunt GaAs FET, which is the most important for low distortion operation of the high power RF switch IC, is a very complicated device to analyze the RF voltage. Because the conventional measurement method has an influence on the behavior of the switch, it has not provided the correct measurement value. In this paper, we have realized a measurement method without touching the surface of the switch IC using EO-probe. As a result we achieved extremely low second and third harmonics of 70.5 dBc and 75.2 dBc, respectively at the input power of 35 dBm by adoptin SPDT switch IC composed of the multi-gate FET for the thru FET and the stacked-gate FET.