The effect of phase transformation and fine particle dispersion on densification behavior of high purity nanocrystalline alumina was investigated. The γ/α phase ratio of the mixture compacts were affected detrimentally by sinterability and phase transformation of α-phase alumina ceramics. The densification was significantly deteriorated with increasing γ-phase owing to faceted pores which are caused by the formation of a colony. The faceted pores were not existed in the α- Al2O3 ceramics with 10% γ-phase. Densification of the 10% γ-phase dispersed α-Al2O3 ceramics was significantly enhanced by rapid transformation of α-alumina. The grain growth was effectively inhibited by a small amount of nanoparticle dispersion for the alumina ceramics. As a result, dense nanostructured alumina can be achieved in the 10% γ-phase dispersed α-Al2O3 ceramics, even sintering at 1400oC for 3 hours.