Quantum-well like molecular resonances have been observed in electron transfer at a model molecule/metal interface: C6F6/Cu(111). Two photoemission measurement reveals a transient molecular anionic resonance located at 3 eV above the Fermi level. This mol ecular resonance is dispersed parallel to the surface, i.e. resonant quantum well behavior. Both the energetic position and the dispersion of this molecular resonance depend intimately on the interfacial electronic structure and the strength of molecule-s urface interaction. With increasing coverage of pre-adsorbed atomic hydrogen which weakens molecule-surface interaction, the position of molecular resonance in C6F6 increases while the effective electron mass decreases. We conclude that the resonant molecular quantum well is confined to the interface and is a result of both molecule-molecule and molecule-surface interaction.