In this study the processes of the free sintering following different compaction methods: magnetic pulse compaction and conventional rigid die pressing of delta-phase alumina and alpha-phase alumina nano-powders is investigated. The purpose of these experiments is to obtain high-density samples of aluminum oxide with ultrafine grains and high strength. The starting powder of delta-phase alumina is obtained by an explosion of an aluminum wire in oxygen atmosphere. The nano-powders of alpha-phase alumina are prepared by heating the starting powder up to 1300 °C, which is above the phase transition temperature. Dilatometry, X-ray analysis, differential thermal analysis and scanning electron microscopy are used in this study. Essential distinctions in the sintering processes are detected in the compacts fabricated by traditional and magnetic pulse compaction.