We developed an electroformed copper lining for RF cavities. The pure copper lining with mirror-finish surface was produced by electropolishing the machined surface of an electroformed copper layer obtained from an acid copper sulfate bath containing no organic additives. The copper lining was successfully applied to large RF cavities for the Japan Hadron Facility being constructed by the High Energy Accelerator Research Organization. Dramatic rise in accelerating performance was achieved by this new lining.