This paper aims to examine the process of the modernization of the Republic of Korea`s Armed Forces during the participation in the Vietnam War. The third Republic of Korea`s government, the Park Chung-hee administration, set the economic growth as its national primary goal. This paper continues to discuss hot issues related to Japan-South Korea normalization and the Status of Forces Agreement between the U.S. and the ROK during the period from 1965 to 1966. Korea pursues its national strategy reinforced by self-reliant defense posture and solid ROK-US alliance throughout the Vietnam war. Especially, Ambassador Brown provided the ROK with his memorandum addressing the security issue and U.S. commitment to the defense of Korea. The Brown Memorandum included ten military assistance items and six economic assistance items. The memorandum was perceived in the negative manner except the economic part. However, this paper explored that the Brown memorandum dictated a great contribution to the modernization of the ROK`s Armed Forces.