In our study, water dispersible magnetite (Fe₃O₄) nanoparticles were continuously synthesized in water under high temperature and pressure in the presence of 3,4-dihydroxyhydrocinnamic acid (DHCA) by using a tubular flow reactor. The prepared Fe₃O₄ nanoparticles were well dispersed in water because the surfaces of the nanoparticles were fully covered by DHCA molecules and the –COOH groups in the DHCA molecules were exposed to the surrounding water. Cytokines such as IL-12 and TNF-α were not produced from the dendritic cells of mice by co-incubation with our synthesized Fe₃O₄. This indicates that the synthesized Fe₃O₄ had no immune stimulating property for the dendritic cells of the mouse. Therefore, our synthesized Fe₃O₄ nanoparticles are suitable for biological applications such as magnetic resonance imaging contrast agents and carriers for drug and gene delivery, and in areas such as hyperthermia therapy for cancer, biosensors, and tissue engineering.