(H2O)6 is the smallest water cluster to form three-dimensional(3-D) structure, and there exist a few low-energy isomers. The stability of the isomers and the isomerization process are studied by ab initio calculations. The difference in energy between the ring structure and the most stable prism is 0.31 eV, which is the energy of one hydrogen bond. The isomerization process of water clusters corresponds to the breaking and/or the reforming of hydrogen bonds. For (H2O)6, the isomerization among the low-energy structures involves the breaking or the reforming of only one hydrogen bond, and the energy barriers separating the isomers range from 0.07—0.21 eV.