Ultrasonic vibration aided electrical discharge machining (UEDM) in gas is a new machining technology developed in recent years. This technology uses air or oxygen as dielectric and ultrasonic vibration is applied to the workpiece during machining. UEDM in gas can avoid environment pollution, the most serious disadvantage of conventional EDM in kerosene-based oil or other dielectric fluids, and it is environmental-friendly. The technology also has virtues of wide applications, high machining efficiency, and simple tool electrodes and so on. In this paper, the formation and transformation of the spark plasma and the mechanism of material removal during a single pulse discharge are introduced.