We report on a systematic X-ray diffraction analysis for the ordered water inside single-walled carbon nanotubes (SWNTs) with diameters of 10.9-15.2 Angstrom. Four distinct ordered structures identified in this diameter range were assigned to polygonal ice-nanotubes predicted by molecular dynamics (MD) calculations. The ordering transition temperature rose from 190 K of octagonal ice-nanotubes to 300 K of pentagonal ice-nanotubes without applying high pressure when the SWNT diameter decreased. The results indicate a crossover from bulk to atomic scale phenomena with decreasing SWNT diameter. (C) 2004 Elsevier B.V. All rights reserved.