Abstract: The improved potassium periodate oxidation spectrophotometry was used to determine the manganese content in sea water and the reagent amount, chromogenic curves, absorption curve, the coefficient of variation and some influencing factors were evaluated. This method was found suitable for determination of manganese with the optimal range of 1 to 50 mg/L in sea water and 1 to 30 mg/L in fresh water, with variation coefficient of 0.31% - 26.61% and recovery efficiency of 76% - 112%.