In this work, Fe3O4 NPs were synthesized using ethylene glycol to reduce FeCl3·6H2O. The TEM image of Fe3O4 NPs (Fig. 1a) shows Fe3O4 NPs are spherical and well-dispersed with a diameter of about 180–200 nm. From the TEM and SEM images of Fe3O4@PDA (Fig. 1b, c), it is obvious that PDA shell layers of about 20 nm in thickness are coated on the Fe3O4 cores, which were formed just by the simple deposition and self-polymerization of DA. In the EDS spectrum of Fe3O4 NPs (Fig. 1d), the contents of C and N elements are very low, however, higher C and N contents are observed for Fe3O4@PDA (Fig. 1e). These results are in agreement with the results obtained from TEM measurements, which further confirms the existence of PDA coatings.