Magnetite Fe 3O 4 nanocrystal clusters (50–60 nm in diameter) with good dispersibility in aqueous medium were prepared by modified co-precipitation method in the presence of surfactants including sodium dodecylsulfate (SDS), trisodium citrate dehydrate (TSCD), and polyacrylic acid sodium salt (PAASS). The influence of surfactant on the formation of Fe 3O 4 nanocrystal clusters was studied. It was found that Fe 3O 4 nanocrystal clusters exhibited almost a super-paramagnetic behavior with very small coercivity and a high magnetization (Ms = 44–56 emu g −1). Magnetite (Fe 3O 4) nanocrystal clusters–polyaniline (PANI) composites were further synthesized via a self-assembly process in the presence of Fe 3O 4 nanoclusters containing SDS as the dopant. It was also noted that the morphology, the magnetic, and the electrical properties of Fe 3O 4–PANI composites could be adjusted by altering the contents of Fe 3O 4 and SDS.