A monolayer of ultrathin sheets of highly hydrophobic graphene nanosheets was prepared on a large area substrate via self-assembly at the liquid-liquid interface. Driven by the minimization of interfacial energy these planar shaped graphene nanosheets produce a close packed monolayer structure at the liquid-liquid interface. This monolayer film shows high electrical conductivity of more than 1000 S/cm and an optical transmission of more than 70% at a wavelength of 550 nm. Interfacial self-assembly of these nanosheets demonstrates a promising route for the application of this novel material in optoelectronics applications.