Since 1945, 13 Americans have been sworn into office as president of the United States. Each president has adopted and developed his own unique decision making style. This paper aims to explore how Truman’s style evolved during his presidency.Truman was an accidental President, chosen by Democratic Party regulars to replace Henry Wallace as Roosevelt’s Vice Presidential running mate in 1944. Roosevelt rarely contacted him, even to inform him of major decisions. The President and Vice President met alone together only twice during their time in office. Therefore, Truman knew little of the detail of American foreign and military policy. He turned to his diplomatic and military advisers, such as Averell Harriman, Joseph Davis, James Byrnes, and Henry Stimson, and followed their advice. President Truman’s serious trouble with Secretary of State Byrnes in late 1945 led him to abandon his early decision making style of relying on advisers.There was ambivalence with respect to Truman’s decision-making style from early 1947 until the outbreak of the Korean war. On the one hand, learning a lesson from the experience of Roosevelt’schaotic decision-making style, President Truman came to place more significance upon both the State Department’s Policy Planning Staff and the National Security Council, both of which were establishedin 1947. Most of the national security policy papers were produced through the NSC’s coordinating process. On the other hand, despite the growing importance of the NSC, President Truman participated only sporadically in the council discussions because he did not want to be captive to the NSC’s decisions. Truman’s reluctance reflected his view of the NSC’s role that, since the NSC’s responsibility was to advise the President, at no time should it force its views on the president’s decision. Therefore, President Truman still relied on informal advice by Secretary of State George Marshall and Under Secretary of State Dean Acheon.When the Korean War broke out, President Truman suddenly began to value the NSC process and depend upon the NSC meeting. Particularly, after July 19, 1950, Truman directed the NSC to meet each week in order to discuss national security policy matters. However, both a first important decision of June 26 to authorize air and naval operations in Korea and a second critical decision of June 30 to send U.S. combat troops were made at the informal meeting held in the Blair House. From Truman’s point of view, the NSC was not appropriate for making prompt decisions. He preferred a small group meetingover the NSC.