When European maritime powers occupied most of sea spaces, they began to trade with Asian countries, and gradually to colonization of large areas of land in the 18th and 19th centuries. In the meantime, the border was invented in the Asia, and a few Japanese intellectuals gradually started to argued about the border of Japan, to recognize Japan's characteristics and advantages as a maritime nation (海國).And they imaged Japan as a maritime nation as a way to make differences from China. So In the 18th century, a few Japanese intellectuals who imaged Japan as a maritime nation no longer imitated the political and military systems of continental nations (陸國, notably China); rather, they took the Western maritime powers as a model to build the nation and expand territories. In this sense, to imagine Japan itself as a maritime nation was a step, or maybe a turning point, in Japan's attempt to de-Asianize (Escape from Asia, 脫亞), also, it was a step in the development of the Asianism (especially of those expansionist thoughts of Asianism).That is to say, the cultural border between Europe and Asia was invented by the modern Japanese intellectuals, at the same time, they break the border of the Asian nations to become the west. So In this paper, I would like to argue that Japan's imagination of being a maritime nation was an important factor in Asianism's emergence in modern Japan. For this purpose, I will examine the discourses of Japanese intellectuals concerned with building up Japan as a maritime nation from the 18th century. And then I will discuss Afred Thayer Mahan's sea power theory served as one of the factors that lead to the emphasis on military overseas expansion in Japan's (expanisionist) Asiaism. At last, as China is urging to get the sea power of the Asia-Pacific region, want to rebuild China as a maritime nation recently. I will discuss the conflict of the two maritime nations in the border of seas.