BaO·6Fe2O3 was produced by hydrothermal treatment of barium nitrite, ferric nitrite and potassium hydroxide under supercritical water conditions. The crystallinity and size of the barium hexaferrite particles were enhanced by increasing the reaction temperature and pressure in the batch system. In the continuous synthesis process, the particle size was decreased by increasing the reaction temperature but was not changed when the reaction pressure was increased. The reaction tempera-ture is a major operational variable for the control of particle size in this rocess. In particular, 0.1-0.2 mm fine particles were synthesized at a pressure of 40 MPa, mixing point(MP2) temperature of 200 ℃, tubular reactor temperature of 400 ℃ and residence time of 100 sec. The particle size distribution was very narrow.