We reported here the preparation of Fe3O4-polypyrrole (PPy) nanocomposites with both magnetic and conducting properties. The magnetic properties of the resulting composites were investigated by vibrating sample magnetometer (VSM). The Fe3O4-PPy nanocomposites had coercive force from 98.4 to 116.3 Oe and saturation magnetization front 0.268 to 9.23 emu/g with the increasing Fe3O4 content. The electrical conductivities ranged from 10(-5) to 10(-2) S/cm, depending on the Fe3O4 content. The average size of Fe3O4-PPy nanocomposites with core-shell structure was about 50 run. Structural characterizations by Fourier transform infrared (FTIR) and thermogravimetric analysis (TGA) proved the interaction between Fe3O4 and PPy chains. (C) 2003 Elsevier Science B.V. All rights reserved.