The authors found CuOx thin film with gradual oxygen concentration (GOC) distribution enhances resistive switching characteristics for nonvolatile memory applications. By using Al/GOC CuOx/Cu structure, not only no forming is needed but also the endurance of switching is greatly enhanced. The device with GOC CuOx demonstrates resistance on/off ratio of greater than 100 and endurance of more than 12 000 cycles. The "forming-free" characteristics appear related to thinner CuO and the endurance enhancement appears related to gradual gradient of oxygen vacancies in CuOx. Thus, the thickness of CuO and distribution of oxygen (as well as vacancies) play important roles for performance improvement. (C) 2008 American Vacuum Society.