Magnetic nanoparticles were synthesized by the addition of ammonium hydroxide to an iron chloride solution by chemical co-precipitation. In order to examine systematically the crystal phase, average size, and magnetic properties of the magnetic nanoparticles, the following were used as experimental parameters: molar ratio of Fe2+/Fe3+, composition of the iron chloride solution, amount of ammonium hydroxide, reaction temperature, and oxidation time of reaction precipitate. In the processing conditions of Fe2+/Fe3+ ratios of 0.5 and 1.0, iron chloride solutions of 0.1-0.8 m, NH4OH molar ratios of 6-14R, reaction temperatures of 25-80 degrees C, and oxidation times of 5-90 min, the co-precipitated nanoparticles were observed to exist as a single phase of Fe3O4. The average size of the particles was approximately 20 nm, and their magnetization was saturated at about 60 emu/g with superparamagnetism. When the iron chloride solution comprised only Fe2+ ions, the oxidation of the reaction precipitates also developed a Fe3O4 phase. However, the particle size reached 78 nm with increasing oxidation times, and the saturation magnetization increased significantly to 82 emu/g while its coercive force was 150 Oe, which indicated that the nanoparticles were paramagnetic.