Abstract: In this paper, the mass spectrometry of three 4,20-double bonded 5/7/6 type taxoids and association of the fragmentation behavior with the hoes and positions of the substituents have been investigated by tandem mass spectromecy the results demonstrate that the loss of HOBz is the major mode of decomposition for M+Na+ ions of 1 and 2, while M+H+ ion of 3 mainly undergoes the decomposition reactions of eliminating one and two HZO molecules. After losses of all AcO and BiO groups and some OH groups, 1 and 2 form fragment ions with stable conjugated systems at ac 237, and 3 at m/z 253.