The World War II broke out in Europe in 1939, and the United States set up the National Defense Research Committee on June in 1940. On April 3,1940, the Meeting of Trustees of the Rockefeller Foundation decided on a grant of $1,150,000 to support the construction of a new 184-inch cyclotron at the University of California. This paper elucidates the process leading up to that decision. The decision-making process can be divided into three stages : The first, beginning in October 1939, saw initial enthusiasm for the giant cyclotron project; the second, lasting until February 1940, involved changes in the Foundation's internal circumstances and limitations on funding; the third, which began in early February 1940, saw specific steps toward the materialization of Foundation support for the project. Doubtlessly, Lawrence's supporters tangibly and intangibly influenced the Rockefeller Foundation's decision to support the construction of the giant cyclotron. The decision-making process, however, seems to shed light on the Foundation's grant-making plans or grant-making policy. That is, the Foundation was deeply involved in drawing up the plans from the start, and provided grants for carefully-selected, large-scale, and long-range projects in the fields it was interested in. The giant cyclotron project, for which single-source support was an important issue, seems to be one such case.