Abstract： Fragmentation behavior of diterpenoids was investigated by ESI/MSn and the qualitative analysis of diterpenoids in the bark of Pseudolarix kaempferi was performed using high-performance liquid chromatography/multi-stage mass spectrometry(HPLC-ESI/MSn).The characteristic fragmentation behaviors of the diterpenoids are the cleavages of the lactone ring and C4-O bond.Furthermore,the eliminations of substituent groups at C-18,C-7 and C-8 can also be observed in the MSn(n = 3-4)spectra.For C-4 acetoxy subsititued diterpenoids,[M+Na-60]+and[M-H-104]-are the base peaks of MS2 spectra in the positive and negative ionization modes,respectively.For C-4 hydroxyl subsititued diterpenoids,[M+Na-44]+and[M-H-62]-are the base peaks of MS2in the positive and negative ionization modes,respectively.For C-18 glucosylated or esterized diterpenoids,[M+Na-44]+is the base peak of MS2 spectra in positive ionization mode.These fragmentation rules were successfully exploited in the identification of diterpenoids in methanol/water(6 : 4)extract of P.kaempferi by LC-MS in positive ionization mode.A total of 9 diterpenoids were identified or tentatively characterized,and one of them is reported here for the first time.The described method could be utilized for the sensitive and rapid qualitative analysis of P.kaempferi.