A series of bicanonical ensemble Monte Carlo (BC MC) simulations has been performed to calculate Na+ hydration Gibbs energy in aqueous solution. The hydration Gibbs energy of Na+ ion in aqueous solution is the difference between formation free energies of Na+ (H 2O)n and (H2O)n clusters at n → α. The convergence of the hydration free energy to bulk water value is fast, and the results at n = 60 turned out to be in good agreement with experimental ones and those calculated using free energy perturbation method [1]. The ion-water interaction has been described by Aqvist's pair potential [1] and SPC model [2] has been used for water-water interactions. The behaviour of the absolute Gibbs energy, the entropy, the internal energy of the clusters and the development of hydration shells' structure with the increase of the number of water molecules are discussed. © 2005 Elsevier B.V. All rights reserved.