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 -53.5 degrees C from the solid to liquid state.