In the present study, the ultra-bright synchrotron radiation X-ray was applied to the observation of small cracks in steels. It is important to investigate the applicability of synchrotron radiation X-ray and several parameters for the observation of defects in steels, which is widely used in industrial structures. The Synchrotron radiation X-ray micro computed tomography (SR–μCT) was applied to the three-dimensional observation of small cracks which were initiated either in fretting or in torsion fatigue test. It was found that phase contrast imaging technique has enabled the reconstruction of clear crack images with small opening displacement of about 1μm or submicron scale. The shapes of surface and inside cracks obtained by SR–μCT were agreed well with those by the scanning electronic microscopy. SR–μCT can also display characteristic inside shape of fretting fatigue cracks those were not perpendicular to the body axis and coalesced each other under surface. Torsion fatigue cracks of 10∼15μm in depth can be also observed in high-strength steel using SR–μCT.
