Abstract: Persulfate oxidation method is the conventional method for the determination of dissolved total nitrogen in seawater.Based on the different ways of nitrate reduction, persulfate oxidation method can be categorized into two types:zinc-cadmium method and cadmium-copper method.Combined with the results of dissolved total nitrogen in seawater measured by 17 laboratories, the comparison of the determination of dissolved total nitrogen in seawater using these two methods was done, and the influencing factors were discussed.The internal-laboratory results showed good precision, and the inter-laboratory comparison results indicated that the zinc-cadmium method was more fluctuating than the cadmium-copper method.For the same seawater sample, lower values were measured by the zinc-cadmium method compared with the cadmium-copper method.Under the condition of using the same experimental reagents and water, the measured concentrations of dissolved total nitrogen with both methods fluctuated obviously, indicating that the impurities from digestion bottle, the nitrate reduction ways and the contamination introduced during the experiment exerted significant impacts on the results of dissolved total nitrogen determination.