Abstract: Ammonia-N is one of the important compositions of marine sediment and the most active component at the sediment-water interface. The routine method uses NaCl, MgCl or KCl and other neutral solvents as extractants. However, only part of ammonia-N in sediments can be determined by this method. A new method to determine ammonia-N in sediment is presented in this paper. Ammonia-N in sediment was extracted by the 0.1 mol/L HCl, then determined by hypo-bromate oxidation. Based on a large number of experiments, the optimal measurement conditions were selected, which included hypo-bromic acid concentration, NaOH concentration in hypo-bromic acid solution, acidity for chromogenic reaction and dosage for sulfanilamide. Almost all of the ammonia-N in marine sediments can be extracted by this new method with better precision and accuracy than alternative methods.