The manufacturing process of micro-grooves inside thin-walled copper heat-pipe is always complicated and difficult. Separately adopted GCr15 and W18Cr4V multi-tooth mandrel, a series of experiments have been done to machine axially inner micro-grooves of micro-heat-pipe with oil-filled steel-ball spinning technology to analyze the mandrel effect and failure mechanism. It was studied how the tooth-shape of multi-tooth mandrel affects the inner micro-groove shape of the copper tube and how the working position of multi-tooth mandrel affects groove-wall surface morphology and groove path. The wear and fracture characteristics of multi-tooth mandrel are introduced. It is proved that the form of GCr15 mandrel failure in machining micro-grooves is tooth-broken or tooth-collapsed, and its service life is short. Differently, the form of W18Cr4V mandrel failure is felt wear, and its service life is long.