Abstract: As technology has developed along with the increasing mechanical demands placed upon it, the need for fatigue exceeding 107 cycles or even longer for machines and components is necessary, not only for safety and reliability, but also for minimizing the economic and human costs brought about by failure. Titanium alloys have been one of the most widely used and most important materials in the aerospace domain owing to their superior properties of high strength-weight ratio and good temperature resistance. Studies have shown that S-N curves of TC4 alloy exhibit a linearly decreasing tendency and no fatigue limit around 107 cycles under very highcycle fatigue. Thus, fatigue strength design according to the traditional standard is adventurous to some extent. In this study, an electromagnetic resonant fatigue testing machine at a frequency of 100 Hz was employed to carry out fatigue tests and investigate the influence of two stress ratios (R = 0. 1 and-1) on TC4 titanium alloy under a very high-cycle fatigue regime. The results show that S-N curves under each of the two stress ratios present the so called"duplex characteristic"while their respective failure mechanisms are different. The fracture of specimens under R = 0. 1 corresponds to two modes, i. e., surface failure induced by machining defects, and interior fisheye failure, accompanied by the appearance of facets. The horizontal part of the S-N curve at stress ratio of 0. 1 represents the transition stress between the surface failure and the interior failure, beyond which the surface failure can take place, while surface failure without facets only occurs under R =-1. Based on fracture mechanics, the threshold value of small crack growth is lower under a positive stress ratio and in a vacuum, which is more conducive to crack propagation and formation of facets. From these results, the interior fatigue failure process and mechanism of TC4 titanium can be explained as follows: (1) the appearance of slip lines or bands inpartial α grain; (2) initiation and coalescence of micro-crack; (3) formation of granular bright facets (GBF); (4) formation of fisheye; (5) unstable crack propagation outside the fisheye; (6) instantaneous fracture.