A novel approach to synthesize core-shell nanoparticles with Fe3O4 as the magnetic core and PPY as the conducting shell was investigated．The Fe3O4 nanoparticles primarily prepared by precipitation-oxidation method

and the Fe3O4-PPY nanoparticles with core-shell structure was synthesized via all in-situ polymerization of pyrrole monomer in an aqueous solution containing well-dispersed Fe3O4 nanoparticles and surfactant NaBS．The particles are almost spherical with diameters ranging from 20～30 nm．These particles are polydisperse and some of them agglomerated due to magneto-dipole interactions between particles．70％ to 80％ of the Fe3O4-PPY particles are ranged from 30～40 nm in diameter．The core-shell structure of nanoparticles was evidenced by TEM observations. The Fe3O4 content was found to affect significantly both the conductivity and the magnetization of the resulting PPY composites．When 9％ Fe3O4 was incorporated into PPY

the conductivity of PPY composite was gready reduced from 6.52 × 10-3 to 1.94 × 10-4 S／cm．Further increasing of Fe3O4 content from 8.2％ to 28.2 wt％

resulted in slight reduction of the conductivity at room temperature．The decrease in conductivity of Fe3O4-PPY composites with increasing Fe3O4 content was due to a decrease in the doping degree

assigned as S/N ratio．The S/N ratios decrease from 0.34 to 0.19 with increasing Fe3O4 content from 0 to 28.2％．The magnetic properties of the Fe3O4-PPY nanocomposites depend on the Fe3O4 content too．Increasing Fe3O4 content from 0 to 28.2％ considerably increased both the saturated magnetization (Ms) and the coercive force (Hc) from 0 to 23.42 emu／g and 0 to 45.2 Oe

respectively．Since PPY powder is not magnetic

the ferromagnetic properties of the composites are attributed to the magnetic Fe3O4 nanoparticles．The magnetic Fe3O4 nanoparticles can improve the thermal stability of Fe3O4-polypyrrole nanocomposites due to interaction between Fe3O4 particles and PPY chains．