This paper systematically reconstructs the path of 'the model of a core of a future city' concept, from formation to culmination, proposed by Uzô Nishiyama's team for the Master Plan of the site of the Osaka World Exhibition of 1970. Although it was not realized, what was important was the proposal to build, in the Expo 70 site, a structure that could be used for the event and which could later become a model for a future city centre area. The main objective of this paper is to clarify fundamental elements of that idea through an analysis of Nishiyama's plans from 1960 to 1973, which define a singular period within his whole career. Nishiyama was an important figure in Japanese urban planning, relatively unknown abroad because few of his plans were realized. His importance lay in his work developing a critique of current planning practices in Japan. Support for his ideas led him to become Vice-President of the Architecture Institute of Japan in 1959 and to be consulted on major planning projects of national importance. The new concepts he proposed during the peak of his career will be contextualized within the historical process of Japanese urbanization, showing that Nishiyama developed his ideas by responding critically to the system of planning then in place. This paper uses, among others, primary sources collected in 'The Memorial Library of Uzô Nishiyama' and aims to introduce Nishiyama's work in detail, almost unknown internationally. Also, this investigation gives a new perspective on the site planning process of Osaka Expo 70, commonly regarded as a project developed mainly by the Japanese architect Kenzô Tange.