The present invention relates to a preparation method of .alpha.-alumina nano powder, and more particularly to a preparation method of .alpha.-alumina nano powder that has a uniform particle shape and size distribution, is capable of being produced at a low temperature, and contains less than 20 ppm of alkali metals such as Na and K. the .alpha.-alumina nano powder is prepared by glycolating aluminium alkoxide in glycol solution containing the .alpha.-iron oxide or .alpha.-alumina nucleation seed and carrying out glyco thermal reaction.