The main characteristic of warfare during the Later Three Kingdom period was the active unfolding of naval battles as much as the land ones. Its major area was southwest shores, centering around Naju, waters off the coast of Jinju in the south seas, and Seosan coast, where Goryeo and (Later) Baekjae fought for naval dominance. Development of maritime clashes were much influenced by the outcome of land campaigns：namely battles of Gonsan(927), Kochang(930), and Ueunju (934). This paper deals with series of wars between the latter part of 920 to 936, particularly from a oceanic supremacy view, while focusing on the causes and effects of naval and land battles. The following summary is the result of such study. The battle of Gongsan, which broke out in September of 927, provided a momentum for Baekjae navy to divert to offensive. As Goryeo and Shilla became politically intimate and supply network was being established connecting south-north in Gyeongsang province region, Baekjae sought ways to thwart this situation. Gongsan battle was the result of such circumstances. In the aftermath of defeat, Goryeo has concentrated its forces to north Gyeongsang inlands. Baekjae, on the other hand, was able to mount more aggressive sea activities so as to take the maritime control of southwest coasts. As Baekjae overtook power in the southwest seas, and fightings frequently occurring in north Geongsang interior, Kochang battle of 930 ensued. Goryeo won this battle, and assumed diplomatic and military superiority in the Gyeongsang locality. Baekjae was in crisis, and attempted a breakthrough by successfully attacking Yesung river basin with its navy. Baekjae surprise attack was a major blow to Goryeo, as its external status plummeting and losing substantial part of its naval capacity, whereas Baekjae recovered its foreign stature and obtained an opportunity to expand its maritime power to midnorthern zone of the west sea. Baekjae’s naval operation to enlarge its sea dominance was initiated with the conquest of Daewoo island, which was strategically located for the penetration into middle inland regions of the west sea. Goryeo was unable to stop Baekjae’s advance because of its weakened naval force, but this situation also motivated Goryeo to enhance sea potential. Goryeo‘s plan included building of sea defence network that connects islands, reconstruction of naval strength, enhancing defence power of Yesung river basin and coastal areas of midnorthern part of west sea. Uenju battle of 934 began during this process. The battle was highly significant in terms of maritime strategy since this event ultimately destroyed Baekjae’s ambition in coastal regions of middle west sea and extension of sea power. After increasing its sea force and maritime defence strength, Goryeo launched its final naval counterattack, which was aimed at resuming oceanic traffic of then-isolated Naju area, and regaining the control of southwest waters. In April of 935, Goryeo force led by general Yoo Geum-pil successfully recaptured Naju, thereby enabling Goryeo to command southwestern seas once again. Moreover, Goryeo embraced Gyeonhwon, king of Baekjae, through sea route, so that the faster unification of Later Three Kingdoms became possible. The fact that Gyeonhwon’s asylum took place through naval operation demonstrates that Goryeo’s sea dominance played a critical role in the destiny of the unification wars. In the midst of fierce combats surrounding the supremacy of the maritimes, Goryeo has continuously operated naval activities that guaranteed sea transportation, which in turn defeated Baekjae challenges and accomplished the unification.