Ceramics used in optical and precision electronic devices. It has the advantages of light weight, heat resistance, and wear resistance. However, ceramics are brittle materials with the disadvantage of low strength reliability. The strength of ceramics depends on surface and internal defects. The defects are formed due to the heterogeneity inside the material during the powder forming process. Thus, the strength of sintered materials depends on the powder process method, and evaluation of the size, shape, and distribution of process-induced internal defects is essential to optimize manufacturing process, and to assure the reliability of ceramics. X-ray Computed Tomography (CT) is a powerful tool for nondestructive 3D observation of the internal microstructure. Recently, synchrotron radiation multi-scale CT has been developed at SPring-8, a research facility in Japan. Compared to conventional laboratory-scale X-ray CT, micro-CT mode enables detailed observation of extremely small defects of several tens of micrometers. It enables us to observe defects at two different scales, and gives us beneficial knowledge for understanding of complicated phenomena during sintering. In this study, the shrinkage process of internal defects in polycrystalline alumina sintered using hot isostatic pressing (HIP) was observed by synchrotron radiation CT to elucidate the mechanism of internal defect removal and the factors affecting it. Alumina granules after spray-dried fine particles (TM-DAR from Taimei Chemical Corp.) were used for sintering. The granules classified into three type; with binder (granule B), binder and MgO (granule BM), and without binder (granule BL). Each sample was sintered in vacuum and then treated with HIP. Before HIP, granule B was classified into three major types of defects: spherical defects smaller than 10 μm, short branching defects, and bowl-shaped defects. In granule BM, similar defects seen in granule B were also found, however, the number of defects was smaller. In granule BL, large branch-like defects larger than 100 µm were found. After HIP, the number of large defects disappeared and decreased in all samples. One of the interesting characteristics of the granule BM was formation of "Saturn-like defects". The changes in defect shape due to differences in the grain size and shape of the raw powder were also clarified, demonstrating the usefulness of HIP treatment for densification of coarse Al₂O₃ from the results of micro- and nano-scale 3D observation.