Abstract: The fragmentation pathways of EGC/GC and EGCG/GCG (two group stereoisomers) were studied using the ion-trap time-of-flight (IT-TOF) mass spectrometer with the advantages of high mass accuracy, high resolution, multistage analysis. Hydrogen/deuterium exchange method was used to elucidate the fragmentation processes. The results show that catechin stereoisomers possess the same fragmentation pathways except for some differences in the relative abundances of product ions， and can not be differentiated even in MSⁿ spectrum. The loss of CO₂ for EGC/GC occurs at B ring and the loss of C₂H₂O involve both A ring and B ring. The ions of ^1,4A^-, ^1,3A^-, ^1,2A^- and M-H-B ring^- are characteristic product ions for EGC/GC, and it can be used to propose the substituent group of A ring through the m/z shift of these ions. The ion at m/z 169 corresponded to the gallic acid anion is characteristic fragmentation of EGCG/GCG, which is helpful for differentiating EGCG/GCG and EGC/GC.