Barium hexaferrite, which have been used as the magnetic material for high density recording media, could be prepared from aqueous suspensions containing ferric nitrate, barium nitrate and potassium hydroxide by supercritical water crystallization method. In this process, the effect of the Fe/Ba and the reaction time on the size, morphology and phase of product were investigated with batch reactor of 35㎤ heated in molten salt bath. The morphology and the phase of the fine particle produced were observed by a scanning electron microscope(SEM) and X-ray diffraction (XRD). In this experiments, we could identify that BaO·6Fe₂O₃ is produced through the path of nonstochiometric reaction and the optimum molar ratio of Fe/Ba is from 2 to 7. The crystallinity of BaO·6Fe₂O₃ obtained at a short reaction time(<5 min)was very good and then the growth of particles were not observed in spite of the reaction time increase. These are due to the rapid hydrolysis, fast chemical bonding and dyhydration reaction under supercritical water condition. Also it is thought that supercritical water crystallization process could be developed as a continuous process as the fact that barium hexaferrite could be successfully synthesized at a very shot reaction time under supercritical water condition is identified.