Titanium alloy (Ti-6Al-4V) has been widely used in biomedical field due to its superior mechanical properties, corrosion resistance. The research investigated dimensional error by micro milling a titanium thin wall, with 2.0 mm height and 0.2 mm thickness. In this study, a finite element analysis was applied to analyze the cause of deformation. The result of the analysis shows that cutting force is the main factor in deformation when the workpiece becomes thinner. Paper review shows that it has been widely discussed up/down milling thin walls. Down milling is easier to produce thin walls with wide cutting parameters. Dimensional error decreased 50 % with 95 m/min cutting velocity, 3 μm/tooth cutting feed, MQL cutting fluid. On the other hand, up milling produces thin walls with bad surface qualities. By observing and analyzing the reason of these surfaces, good surface qualities was produced by up milling with MQL cutting force, 0.09 mm tolerance, 0.3 mm axial depth of cut which was able to measure dimensional error. The measurement results of up milling and down milling with the same parameters show that there are no differences in dimensional error between these two strategies. In this study, a up/down milling thin wall deflection model was established to explain the result of the dimensional error above. At last, an ultrasonic vibration-assisted platform was established which supplied an one-dimensional vibration with 25,000 Hz frequency, 5 μm amplitude. It was proved that with MQL and ultrasonic vibration assisted down milling, normal cutting force and average dimensional error decrease 31%, 28% respectively. With MQL and ultrasonic vibration assisted up milling, transverse stripe was decreased obviously, but could not be removed completely.