The tidal movements historically attracted interests of the human society. It not only provoked intellectual curiosity, but also was related to the daily life. This paper examined Ryu Hwi-mun’s (1773∼1832) tidal theory from the perspective of ‘the reorganization of the tidal theory of Neo-Confucianism.’ Ryu Hwi-mun was a scholar of massive literary works who played crucial parts as an intellect from the late 18th century to the early 19th century. Changnangdapmun, published in 1804, revealed his view on nature, which included the tidal theory. In Changnangdapmun, Ryu set forth his ideas on the cause of the tides, the reason of changes of the tide generating force, and the ways to estimate the time of the ebb and flow. He believed that the tidal movement was related to the movement of sky, the high tide and the ebb tide followed the operation of the moon, and the spring tide and the neap tide followed the operation of the sun. The tidal theory of Neo-Confucianism from Yu Jing (余靖) to Zhu Xi (朱熹) explained the tidal movement in relation to the moon and the changes of tides from the difference of movement of the sun and the moon, and interpret the seasonal changes of the tides through the lens of yin-yang theory. The importance of Ryu Hwi-mun’s tidal theory lies in the fact that it critically succeeded to the tidal theory of Neo-Confucianism from Yu Jing to Zhu Xi. He continuously pursued to reveal the profoundness of yin-yang harmony through the tidal movements. In an effort to review the conventional tidal theory, Ryu incorporated other ideas, including the new tidal theory developed after the arrival of Western Learning, that could strengthen his theory. His goal was to reorganize the tidal theory in a more complete form by compensating the weakness of the conventional tidal theory of Neo-Confucianism.