Abstract: The permanent magnetic BaFe₁₂O₁₉ powder with average particle size of 50 nm was synthesized by a method consisted of independent nucleation and crystallization steps and subsequent heat treatment starting materials from Ba(NO₃)₂, Fe(NO₃)₂ and NH₄HCO₃·NH₂COONH₄. The effects of heat treatment temperatures on the particle size, microstructure and magnetic properties of the powder have been studied by XRD, TEM and VSM techniques. The results showed that the hematite T-Fe₂O₃ was the main phase in the powder at heat treatment temperatures below 650℃ and its amount in the powder decreased with increase of temperature. However small amounts of T-Fe₂O₃ still remained in the powder after being heated at 900℃ for 8 h. BaFe₁₂O₁₉ was formed at about 650℃ and its amount in the powder increased with increase of temperature. It needed more high temperature to attain considerable amount of BaFe₁₂O₁₉ and ideal nanosize BaFe₁₂O₁₉ particle in the powder. The particle size also increased with temperature. Heating at 800℃ for 8 h gave a uniform particle size distribution, and heating at 900℃ for 8 h a typical hexagonal shape and chain aggregation were observed. The specific saturation magnetization strength, specific residual magnetization strength and coercive force all increased with temperatuer from 650℃ to 900℃ and reached 39.83 A·m²/kg, 23.96A·m²/kg and 480 kA/m at 900℃ respectively.