To illuminate the behavior of ion pairs in aqueous solution, both ab initio SCRF MO calculation and MD simulation were performed. The free energy difference of each system was decomposed into contributing components in order to investigate the role of the water in aqueous solution. In aqueous solution, the dominant component contributing to the ion pair stability, is not a solute-solute interaction, but an ion pair-water interaction and the energy change that originated from the rearrangement of the water molecules around the ion pair.