Abstract: The surface of two implant titanium alloys Ti6Al4V（TC4） and Ti3Zr2Sn3Mo25Nb（TLM）were modified into fine grains by ultrasonic impact. The surface fine grains were characterized by TEM. The fatigue fracture surface of long-life specimens were investigated with SEM. The fracture characterics were analyzed. The results show that the dislocation glide is not only the main formation mechanism of fine grains in the surface of TC4 and TLM, but also the initiation mechanism of long-life internal cracks. Multi slip system β phase is advantage to the deformation of α phase in TC4. The trigeminal grain boundary of the converged slip of β phase is usually the core of the micro cracks of TLM. The facet area can be approximately delineated by the scattered slip Facets in the crack initiation of TC4 and the dense slip lines of the TLM’s crack initiation.According to the Murakami formula, the calculated fracture stress intensity factor is slightly higher than which of the untreated specimen.