Some segregation in casting alloys are microstructural defects and degrade mechanical properties of the alloys. Therefore it is important to understand the influence of the distribution of segregation regions on fracture behavior. In this study, tensile test was carried out on an Al-Si cast alloy. Three-dimensional (3D) in-situ observation of its tensile fracture behavior was performed by means of the synchrotron X-ray computed tomography (CT) technique. Some segregation regions have similar density to that of the matrix. Phase retrieval processing, which is one of the phase contrast imaging techniques, enables us to distinguish such segregations based on X-ray CT tomographic images. Fracture path was identified by comparing unfractured and fractured 3D volumes. Grey values of each voxel in CT images correlate to the density of the material after the phase retrieval processing. Examining grey values of voxels on the extracted fracture path, it was found that fracture occurred preferably around the segregation part with high density.