Abstract: Fe₃O₄ magnetic nanoparticles were prepared by means of ultrasonic emulsification and then the Fe₃O₄ magnetic fluid was made by dispersing these particles into water with chitosan as surfactants.The influences of the molar ratio of Fe²⁺ to Fe³⁺,ultrasonic time and the amount of chitosan on the magnetic properties of the magnetic fluid were investigated.The results show that ideal Fe₃O₄ magnetic nanoparticles can be obtained under the condition that the molar ratio of Fe²⁺ to Fe³⁺ is 1:1.5 and the reaction temperature is 70℃ when dropping ammonia.The prepared magnetic fluid has high specific saturation magnetization and stability when the volume of the 1% chitosan solution is 50% as large as the total volume of the mixed Fe₃O₄ solution and the ultrasonic time after dropping chitosan is 7.5 min,which are in favor of coating the magnetic particles with chitosan.