Abstract: The structures of ionic liquids N-ethyl-pyridinium chloride ([EPy]Cl) and N-ethyl-pyridinium bromide ([EPy]Br) were studied by density functional theory (DFT) and B3LYP/6-31+G(d,p) calculations. Structures and interactions in liquid phase were investigated by conductor-like polarizable continuum model (CPCM). The geometries of ionic liquids in gas phase were fully optimized by energy gradient technology and the number of imaginary frequency was obtained through vibrational analysis. The simulated infrared spectra agreed well with the experimental infrared spectra. Natural bond orbital (NBO) analysis was performed to analyze the atomic charges and charge transfers in the monomers and complexes in gas and liquid phases. The interaction between the anion and the cation was mainly electrostatic. There were also some hydrogen bonds between cations and anions in ionic liquids. According to the geometry structures, the magnitude of the interaction energies of [EPy]Cl and [EPy]Br and the NBO analysis results, the interaction between the anion and the cation became more weak in liquid phase because the liquid environment counteracted part of the electrostatic interaction between the anion and the cation.