A simple method for the synthesis of graphene oxide (GO) from expanded graphite using an ultrasonic cleaner is described. Due to the synergistic effect between intercalation (oxidation) and ultrasonication, this method showed strongly decreased demand for acid, time saving/less energy consuming, high productivity and without the release of toxic gas(es) than Hummers' method. Single sheets of GO with layer spacing of similar to 1 nm were obtained in less than 1 h. This procedure was safe and expected to be applicable for the scale production of GO. Both graphene oxide film and reduced graphene oxide (rGO) film showed good bending flexibility.