Monte Carlo method was employed to investigate the filling process of nickel into carbon nanotubes. According to the simulation, nickel atoms around a nanotube firstly aggregated into clusters at the two ends of the tube. Then, the nickel clusters entered the tube, diffused along the tube axis and coalesced into a continuous structure. When the nanotube diameter was larger than 0.76 nm, the continuous nickel structure could transform into an ordered fashion with several concentric layers. By analyzing the radial distribution function, the encapsulated nickel displayed some different features in the microstructure compared to its bulk form. (c) 2007 Elsevier B.V. All rights reserved.