Abstract: It has not yet been reported , how to assay the trace phosphorus,contained in kelp . Through research , we foundthat when the sample was decomposed by HNO3-HClO4 and the system was adjusted to faint alkalescence to eliminate metalelements,the salt of P was oxidized by potassium persulfate . After all of these , based on Mo-blue Ascorbiaccid deoxidotionsystem , the new spectrophotometric method to measure these P was formed . The linearity range of this method was 0～2ug/50ml, callback rate was 97%～101% and RSD% was 1.06%～1.20%. The results of samples assayed was satisfactory.

Key words: Potassium persulfate , Separating by ammonia liquid , Spectrophotometric method , Tiny P