The association of a pair of methane molecules in water has been investigated by molecular dynamics simulations with an optimized umbrella sampling method. The potential of mean force, entropy, and internal energy of the association have been calculated, in which for the first time the energetic contribution of water-water interaction to hydrophobic association was estimated. It turns out that the entropic contribution to the association is attractive at short distances and the energetic contribution of water-water interaction is very little. These results suggest that the number of hydrogen bondings between water molecules little change in the process of hydrophobic association.