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Stress intensity factor of fatigue crack tip after two fatigue cracks had connected it was estimated by conventional method using replication technique under rotating bending load. The stress intensity factor was calculated with the surface crack length which was obtained by replica film observation and aspect ratio which was obtained by some way. In the other hand, stress intensity factor was obtained by converting from crack propagation rate of crack tip using the Paris low by micro computed tomography (μCT) with synchrotron radiation of SPring-8. As a result of the comparing both value, the value by conventional method was larger than the value by μCT method.