Bifunctional composite nanoparticles with simultaneous response toward light excitation and external magnetic field are fabricated by electrostatic adsorption of a single layer of conjugated polyelectrolytes on the magnetic nanoparticle surfaces. Cell imaging is realized through incubation of the composite nanoparticles with human hepatoma cell Bel-7402. Both fluorescence microscopy imaging and flow cytometry analysis verify that the bifunctional nanoparticles efficiently penetrate the cell membranes. Transmission electron microscopy reveals that the nanoparticles are confined in the endosome and show clear signs of particle aggregation. The cellular uptake efficiencies of the magnetic-fluorescent nanoparticles can be enhanced greatly by a magnetic field. In vitro cell viability results indicate low cytotoxicity of the nanoparticles even after 72 h incubation. The current method of fluorescence labeling of nanomaterials by electrostatic adsorption is applicable to a variety of charged nanomaterials that exist with great diversity in chemistry and morphology, which is necessary for estimation of cell toxicity and fundamental understanding of phenomena related to the development of nanomaterial based diagnostics and therapeutics.