Abstract: The fragmentation pathway of raddeanin A was studied by electrospray ionization mass spectrometry (ESI-MS/MS) together with chemical derivatization. The derivative was prepared by raddeanin A and trityl chloride, and then analyzed by ESI-MS/MS. Not only normal fragmentation but also special fragmentation were observed. The mechanism of the special fragmentation was advised. The result indicates that the sequence of sugar moieties in raddeanin A can not be determined only by ESI-MS/MS.