In this study, we performed the bending fatigue test and investigated the influence of strain ratio on fatigue life in TiNi shape memory thin wire. The pulsating plane bending, alternating plane bending and rotating bending fatigue tests were carried. Additionally, we carried out the observation of the fatigue fracture surface by a scanning electron microscope. The behavior of fatigue crack was investigated. The results obtained are summarized as follows. (1) The martensitic transformation (MT) stress of the superelastic thin wire (SE-NT) is higher than that of the SMA thin wire (SME-NT) and the fatigue life of SE-NT is shorter than that of SME-NT. Maximum bending strain at the fatigue limit is the MT starting strain. (2) The low-cycle fatigue life curve in plane bending for SE-NT is expressed by a power function of maximum strain εmax and the number of cycles to failure Nf. The smaller the strain ratio for the same εmax, the shorter the fatigue life. (3) In both the rotating bending and the plane bending, fatigue cracks nucleate on the surface of the wire and one fatigue crack grows preferentially. The region in which fatigue crack propagated is fan-shaped.