High transconductance and high channel mobility diamond field-effect transistors (FETs) utilizing self-aligned Sate FET fabrication process have been operated. The 2 mum gate metal-semiconductor (MES) FET shows the high frequency operation for the first time. The obtained cut off frequency f(T) and maximum frequency of oscillation f(max) are 2.2 and 7 GHz, respectively. It is expected that the diamond MESFET with 0.5 mum gate length fabricated by self-aligned gate process shows 8 GHz of f(T) and 30 GHz of f(max).