We present a study of free energy profile, or potential of mean force (pmf), for the association of K+ to 18-crown-6 in water. The counterion Cl- is included, and all long-range interactions are computed via the Ewald summation method. The present results indicate that the minimum free energy is located at the crown ether center of mass and similar to the complex observed in the solid state or simulated in vacuo. In addition, the crown ether retains its D-3d geometry with a small fluctuation throughout the simulations. The calculated binding free energy, and the corresponding association constant, compare reasonably with experimental measurements made on the same system.