Abstract: Momordica saponin I from Semen Momordicae has been rapidly identified by a combination of ESI-MS n methods in both the positive and negative ion mode. The degradation products Ip and sampgenin Ig of momordica sapoinin I were also analyzed by ESI-MS to confirm the structure of saponin I. The ESI-MS mechanisms of saponin I were also proposed based on the mass spectro data.