A low-absorbance differential cold vapor atomic absorption method is developed for direct determination of trace mercury in seawater. The sensitivity of the method is good at 1 ng mercury per liter and precision is excellent with a relative standard deviation of 13.9% at 12 ng Hg/1 level and 3.2% at 39 ng Hg/1 level respectively. The method is proved to be rapid and simple, and has been applied to the analysis of mercury in seawater from Xiamen Harbor and the Central Pacific Ocean. A low-absorbance differential cold vapor atomic absorption method is developed for direct determination of trace mercury in seawater. The sensitivity of the method is good at 1 ng mercury per liter and precision is excellent with a relative standard deviation of 13.9% at 12 ng Hg / 1 level and 3.2% at 39 ng Hg / 1 level respectively. The method is proved to be rapid and simple, and has been applied to the analysis of mercury in seawater from Xiamen Harbor and the Central Pacific Ocean.