Abstract: Dissolved manganese (Mn) in the ocean is one of the key parameters of international scientific project GEOTRACES,which can be used as a sensitive tracer to discuss the redox variation in the ocean.In this paper,the optional conditions for the method of oxidation of leucomalachite green by sodium periodate to determine trace amount of dissolved Mn in natural waters are reported.In 10 mL system,the pH is adjusted to 4.00.2,the proper concentrations of leucomalachite green,sodium periodate and nitrilotriacetic acid are 35 mol/L,4 mmol/L and 8 mmol/L,respectively.The absorbance of Mn is measured at 615~620 nm after reaction time of 4.00.2 h.The method reported here is free from ionic interferences,and the matrix effects can be overcome by standard addition.The detection limit of dissolved Mn is 0.6 nmol/L (3 standard deviation),with a precision of 6.8% for blank and 2.7% at Mn level of 5.5 nmol/L (n=13) in routine analysis.The analysis of Chinese environmental reference materials (GSB 07-1189-2000) and Canada certified reference seawater material (NASS-6) by the method developed here provide results consistent with the certified values (t-test,P=0.95).There is no significant difference of the vertical profile of dissolved Mn in Kuroshio waters of the East China Sea measured by this method compared with the international published results within the same location (t-test,P=0.95).