In recent years, very high cycle fatigue (VHCF) has become serious issues in the field of mechanical engineering. The VHCF phenomenon is mainly characterized by an internal fracture in cycles over 107–108. This type of fracture has been observed not only in ferrous metals but also in Ti alloys. Especially in Ti-6Al-4V, a remarkable decrease of fatigue strength is evident in internal fractures compared with surface fractures. However, the mechanism of internal fracture is not yet fully understood. The internal cracks seem to propagate through a vacuum-like environment that is shut off from the air.Therefore, the effects of oxidation or gas adsorption on crack growth could be almost ignored in the internal crack. On the basis of this concept, we conducted small crack growth tests in air and high vacuum and observed the fracture surfaces to clarify the effects of vacuum on crack propagation. In addition, we applied a micro computed tomography (µCT) technique using synchrotron radiation to observe the internal crack growth non-destructively. By comparing the crack growth process in vacuum with those of internal crack detected by µCT, the effects of vacuum-like environment on small internal crack growth processes were investigated.