In this paper we present an improved procedure to prepare carbon nanotubes bundles functionalized with magnetite nanoparticles. Carbon nanotubes (CNTs) have been modified by hydrophobic adsorption of a carboxylic acid derivative and the previously synthesized magnetite nanoparticles have been attached to the acid groups. Electron microscopy studies show a high density of magnetite nanoparticles on the surface of CNTs. These modified carbon nanotubes become magnetic and can be appropriately oriented by using external magnetic fields.