Crystal-growth-related microstructures and the length-to-diameter ratio of a single-crystal-type alpha-Al2O3 nanofiber were examined using HR-TEM techniques. The fibers exhibited diameters ranging from 50 to 100 nm and lengths of several tens of micrometers. During thermal treatments, the alumina fiber went through phase transformations similar to boehmite. Therefore, the phase evolution, especially the final theta- to alpha-Al2O3 stage of the phase transformation, may be the determining factor in the microstructural evolution of the nanofibers. HR-TEM techniques were utilized to demonstrate that the single crystals were formed by the coalescence of well-elongated alpha-Al2O3 colonies. The fibers grew in the [1 1 0] or [1 1 2] direction instead of [0 0 1]. A thermodynamic analysis revealed that if the alpha-Al2O3 nanofiber that transformed from theta-Al2O3 behaved in a stable manner, there could be a size ratio limit for the length and diameter of each alpha-Al2O3 colony. The smallest potential diameter was calculated to be around 17 nm. (C) 2010 Elsevier Ltd. All rights reserved.