It is discovered that for an n-gonal ice nanotube built from stacking a single type of n-gonal rings of water, the unit cell consists of two stacked n-gonal rings. In one ring the O-H bonds of water molecules line up clockwise whereas in the other ring the O-H bonds line up counterclockwise. Among the n-gonal ice nanotubes examined, the pentagonal or hexagonal ice nanotube appears to be the most stable.