Threshold voltage shift in high frequency operation of 0.3µm and 0.35µm gate SOI CMOS is experimentally studied, using supply current measurement of inverter chains as test structures. The threshold voltage shift is obtained from the measurement of the leak currents in DC and high frequency condition. For a large supply voltage the electron-hole generation current becomes dominant, resulting in lowered threshold voltage, while the threshold voltage becomes higher than DC case for a low supply voltage. A reasonable relation of the threshold voltage shift and average electric field in the channel is obtained in this study. This method will be useful as a measure of "substrate current" for floating body SOI CMOS.