The origin of the difference of melting points (T-m) between tertiary and quaternary ammonium triflates has been investigated by ab initio and molecular dynamics simulations. Experimentally, T-m of tertiary ammonium triflates are lower than those of quaternary ammonium triflates. In this work, we used [N-122H][TfO] and [N-1113][TfO] as typical models of tertiary and quaternary ammonium triflates, and the mutual arrangement of these cation anion pairs were discussed to elucidate the difference of the melting points of these systems.