Ab initio molecular dynamics simulations have been performed on the water/Pt interface. When the surface is neutral, water is found to form a contact layer directing its O atom toward the surface, i.e., O-down configuration. When the surface is negatively biased, the contact layer shows a significant structural change. The O-down configuration is converted mostly to the H-down configuration. As the surface is biased more strongly, we find that a hydrophobic double layer is formed in the contact layer.