The evolution of the outer lake of Hwajinpo Lagoon in Korea has been reconstructed using environmental proxies (lithologic, geochemical, and fossil data) with a chronology established using 7 accelerator mass spectrometry (AMS) radiocarbon dates. Grain size, water content, and X-ray analyses from the core of outer coastal lakes (HJ99) were used to reconstruct sedimentary environments by using total organic carbon, C/N, S, and C/S chemical proxies. Assemblages of mollusc remains also provided paleoenvironmental information. The environmental changes of the outer lake of Hwajinpo Lagoon can be divided into 6 depositional phases. The basin of the Hwajinpo was exposed and underwent a weathering process before the Holocene period. The muddy sand layer on the weathered bedrock indicated an estuarine system about 6000 BP. The laminated layer implies that the lagoonal system was anoxic between about 5500–2800 BP. The marl layer implies a relatively oxic lagoonal condition with mollusc presence about 2500 B P. The layer of very low sulfur content indicates a freshwater lake system isolated by a sand barrier about 1700 BP. Beginning about 1000 B P, the river system deposits progress progradation on the marl layer. Two erosional landforms could be related with a high standing sea level span during Holocene. These high-stands are dated at 5700 BP and 2200 BP and are supposed to have formed erosional landforms of about 1.6 amsl and 0.8 amsl, respectively. Environmental changes of the outer lake of Hwajinpo Lagoon are considered due mainly to the lake- and sea-level fluctuation during Holocene.