The molecular structures and the intramolecular hydrogen bonding for 1,2-dihydroxybenzene and 2-hydroxythiophenol have been precisely investigated by an ab initio method and density functional theory (DFT), We have considered the several possible conformations with different types of intramolecular hydrogen bonding in given molecules to understand the nature of the hydrogen bonding among these conformers. The optimized geometrical parameters for conformer 1a at the B3LYP levels as well as the computed H-1 NMR chemical shifts for conformer 2b at the RHF/6-31+G*//B3LYP/6-31+G* level are in good agreement with previous experimental results. It is confirmed from these results that the inclusion of electron correlation is crucial to elucidate molecular properties for the intramolecular hydrogen bonding systems, We have also compared the molecular energies between two different conformations both with a hydrogen bond and with no hydrogen bond of a given molecule. In 1,2-dihydroxybenzene, the energy stabilized by hydrogen bonding is about 4 kcal/mol at both ab initio and DFT methods. However, the hydrogen bonding energies are different according to types of hydrogen bonding in 2-hydroxythiophenol: about 1 kcal/mol for conformer 2a with the common type hydrogen bonding and 4 kcal/mol for conformer 2b with the pi type hydrogen bonding.