Core-shell structured nanoparticles with ferromagnetic core (Fe) and shell (Co) were prepared by a chemical reduction method. By adjusting the deposition parameters, the core-shell particles with various Fe:Co molar ratios were obtained. The saturation magnetization decreased with the increase of Cobalt content. The properties of core-shell nanoparticles synthesized under a magnetic field were compared with those prepared without a magnetic field. For the nanoparticles prepared without magnetic filed, the coercivity (Hc) increased with increasing Co content due to the large anisotropy of Co, whereas for the nanoparticles prepared under a magnetic field, the Hc was much lower. The ZFC/FC curves suggested that these particles were ferromagnetic at room temperature. The anisotropy constant K at 340K for core-shell nanoparticle is estimated to be 0.83×105 erg/cm3. The second ferromagnetic phase transition may occur at the temperature lower than 25 K, which led to a drastic change of magnetization at low temperatures.