In this paper, the effects of carbon nanotubes as a support to the morphology and size of Fe 3 O 4 magnetic nanoparticles have been investigated. The synthesis of Fe 3 O 4 /CNTs nanocomposite powder was performed by the direct precipitation method through ferric chloride (II) and (III) at room temperature. The prepared samples were analyzed by X-ray diffraction spectra, Fourier transform infrared spectroscopy, scanning electron microscopy and transmission electron microscopy. The results demonstrated considerable changes in the Fe 3 O 4 nanoparticle size, also the morphology of Fe 3 O 4 /CNTs nanocomposite powder from agglomerative into rode shape.