In order to analyze the influences of process condition on kerosene-based Fe3O4, which are prepared in chemical co-precipitation, orthogonal experiment was used. How process condition influenced the size of nano-particles and saturation magnetization was studied, and the best process condition as 40ml•min-1 NaOH adding speed, 10ml•min-1 sodium oleate adding speed, 45°C reaction temperature, 15min holding time and pH value of 2 of coating sodium oleate was obtained. The performance of magnetic particles was characterized by transmission electron microscopy (TEM) and WSM vibration magnetometer. The diameter of Fe3O4 nano-particles was less than 10nm, saturation magnetization was 2.66×104A•m-1, viscosity of nano-magnetic fluid was 18mPa•s, and density was 1.13×103kg•m-3. This research enhanced the foundation of using nano-magnetic fluid.