Two different sizes of Fe3O4 nanoparticles were synthesized by hydrothermal (H-Fe3O4, approximate to 100 nm) and co-precipitation (C-Fe3O4, approximate to 20 nm) methods. Further, Fe3O4/polypyrrole (Ppy) composites were prepared by polymerizing the conductive monomer, pyrrole on the surface of Fe3O4. In addition, we also synthesized Fe3O4/Ppy/carbon nanotube (CNT) composites by blending the CNTs with Fe3O4/Ppy composites. Epoxy resin-based microwave absorbers were prepared at different thicknesses (1, 2 and 3 mm) by mixing 20% by weight of the as-prepared composites and the complex permittivity (epsilon' - j epsilon '') and permeability (mu' - j mu '') were measured in the 2-18 Gliz frequency range. The composites exhibited significant improvement in microwave absorption (reflection loss <= -10 dB) with a bandwidth from 8 to 12.5 GHz (X-band) by matching the magnetic properties of Fe3O4 and the dielectric properties of Ppy and CNTs for 20 wt% of fillers in 3-mm thickness absorbers. The addition of CNTs into H-Fe3O4/Ppy composites further enhances the minimum reflection loss from -15.8 to -25.9 dB. The present results can pave the way to construct microwave absorbers with a desired reflection loss at a target frequency and thus, the efficient complementarities between complex permittivity and permeability of the nanocomposites can be achieved. (C) 2015 Elsevier B.V. All rights reserved.