In order to study the cutting performance in ultra-precision turning Ti6Al4V assisted by ultrasonic vibration,UTS one of SON-x was used to make single-crystal diamond tool vibrate in the cutting direction,and 350FG Moore Nanotech 4-axis machine tool was used for processing.The experiment was designed according to four factors and three levels orthogonal table.The four factors are cutting speed,cutting depth,feed speed and tool amplitude.Cutting force and surface roughness were used as evaluation indexes.Nexview 8050 of Zygo was used to measure the surface roughness,and Kistler 9256B dynamometer was used to measure the main cutting force,feed resistance,and cutting resistance.The chips were observed by Hitachi TM3000 SEM.The results show that cutting speed has a largest effect on surface roughness and cutting force,while the effect of cutting depth and tool amplitude is less,and feed speed has no significant effect on them.Increasing tool amplitude can reduce cutting force and surface roughness.With the increase of cutting speed,even though the cutting speed is much lower than the critical cutting speed calculated in the traditional formula,the chip also changes from flat bands to regular zigzags bands on both sides,and then develops into spallation chip with cracks in the middle.With the decrease of tool amplitude,the above phenomenon is more obvious.