To provide a positive electrode for a lithium-ion secondary battery, which is highly filled with a positive electrode active material and has a high-density positive electrode active material layer. To provide a lithium-ion secondary battery having high capacity and improved cycle characteristics with use of the positive electrode. After graphene oxide is dispersed in a dispersion medium, a positive electrode active material is added and mixed to form a mixture. A binder is added to the mixture and mixed to form a positive electrode paste. The positive electrode paste is applied to a positive electrode current collector and the dispersion medium contained in the positive electrode paste is evaporated, and then, the graphene oxide is reduced, so that a positive electrode active material layer containing graphene is formed over the positive electrode current collector.