The molecular structures and the intramolecular hydrogen bonding for salicylaldehyde and 2-mercaptobenzaldehyde have been investigated with both ab initio and density functional theory methods. We have considered the several possible conformations with respect to the rotation of two functional groups in a given molecule not only to understand the conformational behaviors but also to estimate the energy of intramolecular hydrogen bonding. The optimized geometrical parameters for salicylaldehyde at the B3LYP levels and the computed H-1 NMR chemical shifts for 2-mercaptobenzaldehyde at the B3LYP/6-31+G* optimized geometry are in good agreement with those of previous experimental data. The results show that the inclusion of electron correlation at the B3LYP levels is more crucial in comparing the relative stability among the conformers of 2-mercaptobenzaldehede than among the conformers of salicylaldehede. The hydrogen-bonding energies are estimated by comparing the molecular energies between two different conformations either with a hydrogen bond or with no hydrogen bond of a given molecule. These energies for salicylaldehyde and 2-mercaptobenzaldehyde are computed;to be about 9 and 2 kcal/mol at the B3LYP levels, respectively.