Cast aluminum alloys have been widely used in transportation equipment because of its high productivity and high specific strength. However, the cast aluminum alloys have low reliability of fatigue strength caused from inevitable casting defect. In our previous work, from the low cycle fatigue test and in situ synchrotron X-ray CT observation of the cast aluminum alloy, it was found that the crack initiation of the cast aluminum alloy was occurred by the fracture of Si particles. In this study, the fracture of Si particle was regarded as the crack initiation, and the geometrical characteristic of fractured Si particle was investigated. The fractured and intact Si particles were identified by chronological CT observation. Geometrical analyses of Si particles were conducted by using high resolution CT images, while Si particles were extracted by an appropriate threshold value of brightness. Geometrical parameters of a few hundreds of thousand Si particles in a specimen were analyzed, then they were evaluated statistically. The effective geometrical parameters were suggested to determine the condition of particle fracture.