A voltage controller to keep the tool tip in the resonance state in ultrasonic vibration assisted drilling process is designed and implemented. An equation relating the overhang of the tool, its material properties and frequency of the vibration actuator is derived. The resonant frequency is obtained by properly tuning the output voltage of the power unit via adjustment of the inductance setting of the unit before the start of the machining process. The tuned voltage is taken as the reference voltage. It is compared with the working voltage monitored constantly during the machining process. Once an unusual variation of voltage is detected, the output voltage of the power unit is adjusted so that the resonance state determined previously can be resumed. Experiments on drilling of Inconel 718 superalloy are conducted to verify the derived equation and the developed controller. It is found that satisfactory results are obtained. The resonance state at the beginning of drilling can be achieved in 12 seconds. The number of holes that can be drilled is increased from 35 to 40 holes per drill. In addition, the surface roughness of the drilled hole is improved, and its value is changed from Ra 2.9 mu m to 2.3 mu m.