The authors have performed micro computed tomography (μCT) with synchrotron radiation of SPring-8 to evaluate the fatigue crack propagation behavior inside specimens. A double-spindle type compact rotating bending fatigue testing machine was developed for fatigue testing by the side of a beam line of SPring-8. Preparatory experiments demonstrated that the resulting data by the machine developed were consistent with the data by conventional Ono-type rotating bending fatigue testing machine. In this study, results from μCT technique were compared to the well-established replication technique in order to confirm the applicability of μCT to the study of fatigue crack propagation behavior. Fatigue crack propagation curves and da/dN-ΔK relation obtained by μCT technique corresponded to the data by replication technique on the surface of the specimens, however, it was also shown that the replication technique without information on inside would not always present relevant information on the crack behavior.