We report a chemical vapor deposition (CVD) synthesis of ultra-large area monolayer graphene films and roll-based layer-by-layer transfer onto flexible substrates. The monolayer shows the sheet resistance as small as ~125 Ohm/square with 97.4% optical transmittance and a unique half-integer quantum Hall effect indicating the high-quality of graphene films. The 30-inch scale multiple roll-to-roll transfer further enhance the electrical properties of the graphene films, resulting in ~40 Ohm/square sheet resistance and ~90 % transparency comparable to recent commercial transparent electrodes such as indium tin oxides (ITO).