The intramolecular hydrogen bonding of 2-nitroresorcinol and 4,6-dinitroresorcinol is investigated by both an ab initio method and density functional theory (DFT). We have considered several possible conformations in which the hydrogen bonding is hindered by rotation of the hydroxyl groups in order to see how much molecular structures are affected by hydrogen bonding. The results show that the geometries of the benzene rings in nitroresorcinols are somewhat changed by intramolecular hydrogen bonding. Geometrical parameters calculated from ab initio and DFT calculations at the higher levels of theory are also in good agreement with data of electron diffraction experiments. On the other hand, the planarity of a nitro group is very responsible not only for hydrogen bonding but also for nonbonding interactions between two oxygen atoms in these compounds. We have also estimated the energy of hydrogen bending by comparing the molecular energies between two different conformers of a given molecule. The magnitude of computed values from both ab initio and DFT methods is about 10 kcal/mol per hydrogen bond.