Abstract: To investigate the fragmentation mechanism of fluroquinolones, six compounds of fluroquinolones were analyzed using electrospray ion trap mass spectrometry by collision induced dissociation in a multi-stage MS full scan mode. The samples were infused into the ion source with a syringe pump and the mass spectrometry was operated in the positive mode. The mass spectra and structures of the six fluroquinolones were compared with each other and it was observed that fluroquinolones gave characteristic fragment ions by the neutral loss of CO₂, HF and CO, corresponding to the carboxy, fluorine and 4-carbonyl group in their structures. There would be another fragment ions by the elimination of CH₂＝CH-NH₂ if the structure was with a 7-piperazidine substituent. These characteristics can be used for future structure elucidation in studies of fluroquinolones and analogue compounds.