Abstract: The NPT ensemble simulation of the difference between chemical potentials in infinite dilute aqueous solutions and that in pure components i at given T, P conditions, and activity coeffidents for methane and benzene in infinite dilute aqueous solutions was carried out in terms of the thermodynamic integration with a coupling parameter, and the preferential sampling method. The TIP4p and OPLS potentials were used for the descriptions of molecular interactions of water and organic substance, respectively. The Ewald method was incorporated into the long range correction for the interaction between water molecules. The simulated result sare in fair agreement with experimental data within the errors for MC computer simulation,Which indicates that our method can be applied to the predication of the properties for organic substances in infinite dilute aqueous solutions.