Abstract: A 1.5-meter long sediment core was collected from the estuary of Bang Pakong River, Bangkok Bay, in July 2015. Calculated with ²¹⁰Pb and ¹³⁷Cs, the sedimentation rate is 2.3cm/a. It means that the sediment has been deposited in the year from 1948 to 2015. Heavy metal element (Cd, Pb, Co, Ni, Cu, Cr and Zn) concentrations, measured by ICP-MS, are 0.11, 29.60, 19.78, 44.18, 28.99, 80.93 and 99.01 mg/kg, respectively. Amounts of Cd, Zn, Pb and Cr increased significantly during the period. The degree of heavy metal pollution ranks in an order of Cd>Zn>Pb>Cr>Cu>Ni>Co. Calculated geo-accumulation indices demonstrate that the sediment are moderately polluted by Cd, and slightly by Pb, Co, Ni, Cu, Cr and Zn. The potential ecological risk indexes of single element(E_rⁱ) are ranked in an order of Cd > Pb > Cu > Ni > Co > Cr > Zn. Ecological risk evaluation of heavy metals suggests that Cd is high to very high in ecological risk, while Pb is low to moderate. The concentrations of Cd, Zn, Cu and Pb in the sediments are lower than the effects range low(ERL), and the concentrations of Ni have exceeded the effects range low(ERL), but below the effects range median(ERM). Cd, Cu and Zn may derive from metallurgical industry and transportation. Cd has an unknown source of pollution, strongly influenced by human activities. The anomaly high Cr may be from textile industry. Inter-elemental correlation of heavy metals before the year of 1990 was worse than that after 1990, indicating that there may be more complex sources of heavy metal pollution after the year of 1990.