The elements of design of high temperature LWR operating at supercritical pressure are described. It includes conceptual design of fuel, core, plant, safety, control and start-up systems. The concept is developed at the University of Tokyo by computer analysis. The feature of the reactor such as economic improvement and hydrogen production potential are described as well as the view from the theory of boiler innovation. The technical background of the concept is LWR and supercritical fossil-fired power technologies. The concept was selected as one of the Generation 4 reactor. The research and development in Japan and in the world are underway. Potential of further design improvement exists.