We have made an estimation of the interfacial tension of octane/water system based on the molecular dynamics simulations, The interfacial energy is the surface excess of internal energy, while the interfacial tension is the surface excess of Helmholtz free energy whose evaluation is generally difficult. The potential of mean force (pmf) of solutes gives the intermolecular interaction which includes the Helmholtz free energy change due to the rearrangement of solvent molecules; then it is expected to give the interfacial tension. We calculated the stabilization energy of an octane pair using the pmf, the decrease of the solvent-accessible surface (SAS) area of the octane pair for contacting, and the value which is the stabilization energy of the octane pair divided by the change of the SAS area with some corrections, and it should correspond to the interfacial tension. The calculated value was 57 +/- 11 erg/cm(2) which is close to the macroscopic interfacial tension of 50.8 erg/cm(2).