Abstract: Ultrasonic vibration assisted micro milling is a kind of ultrasonic vibration, exerting a certain frequency and amplitude on the tool or the workpiece during the micro milling process, which can change the material removal mechanism and improve the micro milling characteristics. Taking Steel 45# as an example, the effect of grain size on ultrasonic vibration aided micro milling results was investigated, the orthogonal experiment was used to study the micro-milling of 45# steel with different grain sizes. The effects of grain size and ultrasonic vibration on milling force, dimensional accuracy and surface roughness were analyzed in the orthogonal experiment. It is proved that the material with large grain size corresponds to the smaller milling force under the same process parameters, and the material with larger grain size can obtain better processing surface quality.