Water nanotube clusters (WNTs) contained in nanochannels of molecular porous crystal are prepared. The structure of these WNT is almost the same as that of partial clathrate-hydrate type-I, which is constructed from 1-D {5 12 6 2 } n water clusters stabilized under ambient atmosphere. The WNT has a structural phase transition at ¹53.5°C from the solid to liquid state.Investigation of some water-molecule clusters formed in the interstitial space of supramolecular crystals has attracted attention as an important method to understand the behavior of bulk water and ice.1 However, since most of the water clusters are actually small in unit cell and stabilized by direct H bonding with supramolecules, they display no macroscopic behavior of bulk water such as a phase transition and a glass one. We have constructed huge water clusters in porous molecular crystals with one-dimensional nanochannels by self-organization of designed molecular building blocks.2 The water nanotube clusters (WNTs) in the channel spaces show a structural phase transition, and the crystal structure of WNT corresponding to the melting state is also observed. Interestingly, the crystal structure of WNT, stabilized by H bonds with porous outer walls is constructed from a partial structure {5 12 6 2 } n of type-I clathrate hydrates. In a previous study, WNTs with an infinite tube-like structure have successfully been confined to channel cavities of a nanoporous crystal { 3+ and TMA 3¹ in water, and the characteristics are compared. The characteristics of the WNT generally change with a change in molecular building blocks forming the nanoporous framework. Figure 1 shows the results of the differential scanning calorimetry (DSC) measurement of 2. The melting temperature of WNT confined to nanometer-scale regions with diameters greater than ca. 2 nm usually conforms to the Kelvin equation, which shows the capillary freezing point depression 5 and decreases with a decrease in the diameter of the region. The melting temperature of the WNT of 2 is ¹53.5°C. It can be maintained in the liquid state in the form of stable supercooled water down to temperatures lower than those required for maintaining the WNT of 1 in the liquid state (¹28°C). Most water molecules in the WNT affect strong interactions of the WNT with the outer wall of the nanoporous channel. Therefore, the tendency of the melting temperatures of 1 and 2 to shift toward lower values is probably attributed to the difference between hydrophilic interactions occurring on the channel surface (which are affected by channel sizes) and hydrophobic interactions occurring on the channel surfaces of CNTs.6 One of the reasons why the WNT of 2 is maintained in the formation of supercooled water down to a low temperature of ¹53.5°C has been revealed in a previous study of bulk water, in which imaging-plate X-ray diffraction of supercooled water was carried out. 7 The model contains two water molecules in one pore of a 5 12 6 2 tetradecahedron unit. This model is comparable to the aggregation of water molec...