Ni nanotube arrays with different diameters were fabricated in the pores of the porous anodic alumina membranes by direct current electrodeposition. The crystal structure and micrograph of Ni nanotube arrays were characterized by X-ray diffraction, transmission electron microscopy, and field-emission scanning electron microscopy. The results indicate that Ni nanotubes have no preferred orientation and are polycrystalline structure. The magnetic behaviors of Ni nanotube arrays with different diameters are investigated, and the coercivity of Ni nanotubes depends strongly on their diameters. The size-dependent behavior of the coercivity is qualitatively explained in terms of localized magnetization reversal. (C) 2010 Elsevier B.V. All rights reserved.