The fragmentation behavior of isoflavones was studied using electrospray ionization-ion trap-time of flight mass spectrometry （ESI-IT-TOF-MSn）. It was found that the isoflavone glycoside bond was easily broken. The fragmentation occurred mostly on the C-ring, and the fragment ions of A^1,3＋ produced by the RDA cracking will predict the hydroxylation replacement on A-ring or B-ring. In addition, four carbonyl groups on the C-ring were fragmented through neutral loss of 28 （-CO）. A and B-rings primarily lose substituents which including a neutral losses of 32 （-CH3OH）, 16 （-CH4）, or 16 （-O）, and 18 （-H2O）. A-ring in the presence of adjacent hydroxylation, also easily made to be a neutral losses of 28 （-CO） or 18 （-H20）. It is likewise common to see methoxy replaced with a neutral losses of 16 （-CH4） or 32 （-CHaOH） in B-ring, also the hydroxylation on benzene ring can occasionally results with the neutral loss of 28 （-CO）.
