A new machining method, ultrasonic vibration aided electrical discharge machining (UEDM) in gas, is proposed in this paper. It is shown that electrical discharge machining with ultrasonic vibration aid can be achieved well in gas medium. In the process of UEDM in gas, the tool electrode is formed to be thin-walled pipe, high pressure gas medium is supplied from inside, and ultrasonic vibration is applied to workpiece. The property of ultrasonic is introduced. The same to other sound waves, ultrasonic have the characteristics of reflecting, refracting, intervening and resonance. The mechanism of elastic pole keeping in resonance with ultrasonic transferring from one end to the other end is explained with characteristics of ultrasonic. During the process of UEDM in gas, ultrasonic vibration of workpiece can improve the machining process. The theories of ultrasonic vibration increasing materials removal rate (MRR) are introduced. One theory is that the adhere strength between the metallic liquid drops and workpiece is not enough for the accelerative vibration, so metallic liquid drops will be ejected off easily. Another theory is that ultrasonic vibration increases the number of the effective discharge.