Abstract: Tiny burrs are the key influence factors on the forming quality of micro parts,which are generated in the micro cutting process.In this paper,a horizontal ultrasonic vibration was superimposed on workpiece by double-slotted rectangular hexahedron sonotrode.The ultrasonic micro milling experiments with horizontal workpiece vibration were applied to machine micro grooves on 304 Austenitic Stainless steel sheets and the burr forming characteristics of ultrasonic micro milling process with different experimental parameters was studied.The experimental results show that the horizontal ultrasonic vibration of workpiece can change the burr shape from elongated flocculent structure to continuous flake structure,which is located on the top of micro grooves.With the increasing of ultrasonic vibration amplitude,the height of flake burr decreases gradually and turns into tiny fragments.On the other hand,the horizontal ultrasonic vibration of workpiece can also be helpful to reduce the burr length of micro milling groove. When the tooth feed rate of micro-milling cutter is slightly larger than its minimum cutting thickness,the decrease of burr length will be the most obvious.