Ba-ferrite ($BaFe_{12}O_{19}$) nanoparticles were synthesized by chemical coprecipitation method in an aqueous solution. The particle size and the crystallization temperature of the Ba-ferrite nanoparticles were controlled varying the precipitation temperature. The precipitate that was prepared at $0^{circ}C$ showed the crystal structure of Ba-ferrite in X-ray diffraction when it was calcined at the temperature above $580^{circ}C$, whereas what was prepared at $50^{circ}C$ showed the crystallinity when it was calcined at the temperature higher than about $700^{circ}C$. The particle sizes of the synthesized Ba-ferrite were in a range of about 20-30 nm when it was prepared by being precipitated at $0^{circ}C$ and calcined at $650^{circ}C$. When the precipitation temperature increased, the particle size also increased even at the same calcination temperature. The magnetic properties of the Ba-ferrite nanoparticles were also controlled by the synthetic condition of precipitation and calcination temperature. The coercive force could be appreciably lowered without a loss of saturation magnetization when the Ba-ferrite nanoparticles were prepared by precipitation and calcination both at low temperatures.