Two types of fine (0.2 μm) and pure (99.99%) alumina powders are dispersed in ion-exchanged water and compacted into a columnar shape of φ 8 × 60 mm by the High-Speed Centrifugal Compaction Process (HCP) at 11,500 rpm for 3 ks. Specimens are sintered at 1423-1823 K for 5.4 ks. HCP alumina shows good compactibility regardless of starting powder. Its behavior is slightly affected by the dispersing condition of the slip. The sintering behavior of compacts can be evaluated by grain-size/relative-density trajectory. The optimum mechanical properties are realized at a characteristic part of the curve, where the curve starts to go upward. Bending strength of the alumina with different starting materials obeys basically one equation. So does the Vickers hardness. Therefore, the strength-microstructure diagram has validity for HCP alumina of various powders. The powder which gives lower position for grain-size/relative-density trajectory of sintered alumina gives higher mechanical properties.