Conducting and superparamagnetic monodispersed Fe 3O 4–poly(3, 4-ethylenedioxythiophene) core–shell composite nanoparticles with the size about 15 nm, were synthesized via an in situ chemical oxidative polymerization in the micellar solution of lignosulfonic acid (LSA). Core–shell nanocomposites composed of iron oxide (Fe 3O 4) nanoparticles and conjugated polymer, poly(3, 4-ethylenedioxythiophene) (PEDOT), were successfully synthesized from a simple and inexpensive in situ chemical oxidative polymerization of EDOT with Fe 3O 4 nanoparticles in the micellar solution of lignosulfonic acid (LSA) which serves as both a surfactant and a dopant. These nanocomposites (Fe 3O 4–PEDOT) were subsequently characterized for morphological, crystalline, structural, electrical and magnetic properties by high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), four-probe meter and superconductor quantum interference device (SQUID), respectively. Results show that the nanocomposites have a spherical core–shell shape, are ∼10 nm in size and are superparamagnetic with good magnetic saturation and good electrical conductivities. Existence of Fe 3O 4 in the nanocomposites was confirmed by using Energy dispersive X-ray photoelectron spectroscopy (EDAX) and X-ray photoelectron microscopy (XPS). We also investigated a possible formation mechanism of the core–shell nanocomposites, and the effect of Fe 3O 4 nanoparticles on the electro-magnetic properties of the nanocomposites. Such novel conducting and superparamagnetic composite nanomaterials can be applied to sensors, magnetic data storage, electro-magnetic resonance wave absorption, etc.