In recent years, ultrasonic-assisted machining (UAM) has been widely implemented to improve the performance and element quality of machined products. This paper comprehensively reviews the ultrasonic vibration effects on such ultrasonic vibration-assisted (UVA) processes as hobbing (UVAH), lapping (UVAL), electrochemical gear machining (UVAEGM), honing (UVAH), and grinding (UVAG), respectively. Compared with the conventional machining, the UAM significantly reduced the surface roughness, improved the surface microstructure, and quality. Considering the UAM applications surface modification and surface strengthening, the available technologies of ultrasonic impact, ultrasonic shot peening, and ultrasonic deep rolling were analyzed for gears and similar components. It was concluded that the surface was strengthened under UAM through introducing the high-amplitude and large-depth residual compressive stresses, restraining crack propagation to a certain extent, and improving the gear fatigue resistance. Finally, the ultrasonic machining effect on gear fatigue resistance was theoretically substantiated from the ultrasonic vibration system and surface integrity standpoints. This review aims to optimize the application of ultrasonic-assisted machining, in order to produce gears with enhanced fatigue resistance and surface integrity.