Abstract: The derivatizing reaction between OPA and reduced glutathione (GSH) were investigated using ESI-MS, spectrometric analysis.Because of Cannizaro reaction of OPA in the solution, two derivitized products had been formed with conjugated tricyclic structure and bicyclic structure, in which much more fluorescent emmision properties was observed by derivatized product with tricyclic conjugated structure.In PBS buffer soluiton, using OPA as derivatizing agent, GSH content in Sargassum siliquastrum, Silvetia siliquosa, Gracilaria lemaneiformis had been determined by fluorometryic analysis.In OPA+PBS mixture buffer solution with excitation wavelength at 350nm, the sample emmited strong fluorescence at 428nm, the detection limit was 3.6×10-8mol/L.GSH contents determined with standard addition methods in the samples were 0.0714mg/g (Sargassum siliquastrum) , 0.1183mg/g (Silvetia siliquosa) , 0.1970mg/g (Gracilaria lemaneiformis) respectively, and the reconveries were in the range of 99.22%～100.41%.