Growth characteristics of the crystallite and the agglomerate during theta --> alpha-Al2O3 phase transformation of ultrafine (nano-sized) alumina powders were investigated, using X-ray powder diffraction analysis, BET techniques, differential thermal analysis (DTA), and transmission electron microscope observation of the quenched samples. The DTA profile of phase transformation is composed of two portions, nucleation and crystallite growth, which correspond directly to the formation of alpha-Al2O3. The area of each portion and the nucleation temperature were correlated to the measured size of theta-Al2O3 crystallite (Scherrer formula). The maximum crystallite size in the powder system of theta-Al2O3 before transformation is approximately 20 nm. The nucleation can be achieved with the transformation of one theta-Al2O3 crystallite to one alpha-Al2O3 crystallite, at about 17 nm. These alpha-Al2O3 crystallites then grew drastically to a size of 45-55 nm. This can be considered as the primary crystallite size, from which point the agglomerates or polycrystals alpha-Al2O3 are formed with sizes ranging from several tens to larger than 200 nm (BET technique). It is noted that the growth of the ultrafine alpha-Al2O3 crystallites is possibly carried out through the coalescence of the newly formed alpha-Al2O3 nuclei.