Molecular-dynamics simulations were used to investigate the structures of hydrogen molecules formed within (5,5), (9,0) and (10,10) single-walled carbon nanotubes (SWNT). In the (5,5) and (9,0) SWNTs, H-2 molecules preferably form linear lines and helical curves, showing the strong confinement of small-diameter SWNTs. The structure of H-2 molecules in the (5,5) and (9,0) SWNTs is different. showing the dependence on the helicities of the SWNTs. The accumulation process of H-2 molecules in the (10,10) SWNT is also presented, showing the formation of H-2 molecules cylindrical shells. (C) 2002 Published by Elsevier Science B.V.