Clay minerals from the tidal flat sediments along the coast of Youngjong Island, west of mainland Korea, were studied to ascertain their potential as an indicator of sea-level fluctuation. Semi-quantitative analysis of their clay-size fractions by XRD shows that the vertical distribution of clay minerals can be separated into four clay mineral assemblage units. Based on the relative contents of chlorite, four sedimentary units are distinguishable, and the abundance of chlorite is inversely related to that of smectite. The kaolinite content is constant, and so it is clear that chlorite originating from the Han River was transformed to smectite by pedogenesis during regression periods. Therefore, the relative contents of clay minerals from the unconsolidated materials in the tidal flat of Youngjong Island can be used as indicators of transgression and regression in accordance with the sea-level fluctuation.