The purpose is to investigate the effects of the ultrasonic vibration on dimensional accuracy in micro milling process. Several slotmilling experiments were conducted with aluminium alloy as the workpiece material. The desired vibration was proposed from the workpiece side by an ultrasonic vibrator. The width of slot produced by end milling was precisely measured. A one-factor experiment was conducted at first to analyze the variation of slot width before and after ultrasonic vibration was applied, and the experimental results indicated that ultrasonic vibration made contribution to the decrease of size deviation and larger amplitude tends to reduce the slot width oversize. A full-factor experiment was carried out accordingly based on Taguchi method to optimize the parameters involving spindle speed, feed per tooth and vibrating amplitude. 