In this work, we controlled the magnetic properties and water-solubility of CoFe₂O₄ nanoparticles by adjusting the reaction conditions during the preparation process. The growth condition-dependent crystalline, microstructure and therefore their magnetic properties were investigated in detail. It is found that the crystalline and the size of the nanoparticle become larger with the increasing of both the reaction temperature and reaction time. Consequently, the saturation magnetization (Ms) of our products was highly improved. Specifically, the typical sample prepared at 180 ^oC for 2 h, which has a higher Ms and good solubility in water was chosen as an alternative agent in magnetic resonance imaging The results show that it is an effective T₂ weighted MRI contrast agent.