A theoretical model based on the competition between hydrogen-bonding energy and strain energy was constructed to explain the size of native cellulose I beta. The cellodextrins in native crystalline cellulose I alpha and I beta are unusually stable compared to other polysaccharides, not easily prone to hydrolysis even though they are only nanometers in diameter. The stability of crystalline cellulose I beta is most likely due to its greatly enhanced hydrogen-bonding (1113) network. We carried out ab initio calculations to determine the native crystalline cellulose I beta atomic and conformational. structures. For crystalline cellulose, we found that every hydroxyl group in the cellulose structure is hydrogen bonded as both a donor and an acceptor. This agrees well with published X-ray and neutron diffraction data. We also determined the electronic structures and the energetics for one cellodextrin chain, one to four sheets of cellodextrins in cellulose, and the bulk cellulose I beta.