A systematic investigation of water filling of carbon nanotubes is presented. By means of a molecular mechanics approach it is shown that water enters spontaneously into the nanotubes and fulfill them completely. The absorbed water forms single-file structure into narrow nanotubes and ice-like structures into larger nanotubes. The relation between the water occupation and the nanotube diameter is linear when ice-like ordering of water is present. It is found that the number of stored molecules decreases quadratically with the increase of temperature and for larger nanotubes a transition from ordered ice-like to liquid is observed when temperature reaches 330 K. (C) 2012 Elsevier B.V. All rights reserved.