In the study three methods of persulfate digestion for dissolved organic nitrogen (DON) analysis were compared. The recoveries were calculated by using kjeldahl?method for the total dissolved nitrogen (TDN) and isotope ratio mass spectrometry (IRMS) for 15N. The results showed that the Doyle methods obtained higher recovery. Therefore the pre-treatment method of analysis of DON was recommend at the condition of the volume ratio of oxidizing reagent (50 g/L K2S2O8, 16.8 g/L NaOH, 30 g/L H3BO4) to extracts was 1︰1 and the digest is performed at temperature 120℃ for one hour.