Ni chains assembled by Ni spheres have been synthesized via a polymer-free hydrothermal approach. The products were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectrometry (XPS). In order to investigate the formation mechanism of the Ni chains, a series of assistant experiments have been performed. These experiments demonstrated that the pre-formation of Ni(C4H2O6)(2-) and the presence of high concentration of NaOH were two important factors for the formation of Ni chains. Based on the results of assistant experiments, a growth mechanism for Ni chains was proposed. The magnetic property investigation of the obtained Ni chains illustrated that they have ferromagnetic behavior and significantly enhanced coercivity. Such a simple hydrothermal process can be expanded to fabricate chains of other materials. (C) 2009 Elsevier B.V. All rights reserved.