Abstract: In this study, water-dispersible magnetic iron oxide (Fe₃O₄) nanoparticles were synthesized with solvothermal method. The nanoparticles were characterized with a transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM). The in vitro magnetic resonance response and photothermal conversion characteristics of the nanoparticles were evaluated. In addition, the cellular uptake, cytotoxicity and biodistribution were studied. Finally, magnetic resonance/photothermal dual-modal imaging effect of the as-synthesized Fe₃O₄ nanoparticles was investigated in the tumor-bearing mice. The results showed that the obtained magnetic nanoparticles were uniform with a mean diameter of about 125 nm. Moreover, the superparamagnetic Fe₃O₄ nanoparticles showed remarkable magnetic resonance response and photothermal conversion properties. The results of cellular experiments showed that the cell viability was nearly 85% even the concentration of the nanoparticles was up to 1 000 μg·mL^-1, an indicator of good biocompatibility. In addition, the nanoparticles could be taken up by the tumor cells and then located in the cytoplasm. After intravenous injection, the nanoparticles were tended to enrich in the tumor over time, which is helpful in achieving dual-modal magnetic resonance/photothermal imaging. In sum, the obtained Fe₃O₄ nanoparticles showed great potential to be applied for multi-modal bio-imaging which may play an important role in the diagnosis of tumors.