A series of cobalt (Co) nanoparticles were synthesized by employing dendrimers with a trimesyl core as templates and sodium borohydride as a reducing agent. Extensive characterizations of the produced materials were studied using TEM, selected area electron diffraction, energy-dispersive X-ray spectroscopy, FT-IR spectra, TGA, and XPS. And their magnetic properties, including hysteresis loop, field-cooled and zero-field-cooled curves, were investigated by superconducting quantum interference magnetometer. The nanoparticles exhibit supermagnetism at room temperature, accompanied by magnetic hysteresis below the blocking temperature. Co nanoparticles exhibited an excellent T-1 MRI enhancement effect and can be a potential MRI contrast agent. (C) 2010 Elsevier B.V. All rights reserved.