Abstract: Time of Flight Mass Spectrometry (TOF MS), with different electron energy for EI and different gas pressure for CI, of half aquo complex of levofloxacin lactate (LL) were studied. There was no molecular ion of LL as well as its half aquo complex in both EIMS and CIMS, but molecular ions with abundance of 100% of levofloxacin(LVFX) in both EI and CI + mass spectra were appeared. A prominent fragmentation rout of LVFX was an elimination of CO 2 from molecular ion at m/z 361, forming cation A at m/z 317, followed by the cleavage of piperizine ring creating caion B at m/z 246 and C at m/z 71. Further fragmentation pathway was the formation of cation D at m/z 231 from B. All the abundances of fragment measured under 25 eV in EI were greater than those under 70 eV. For CIMS with normal gas pressure (0.01 Pa), the abundance of + ion appears as 100%, whereas all of abundances of fragment ions were very small. When the gas pressure were reduced to 1×10 -3 Pa, the abundance of + ion were reduced to 34%, whereas all of abundances of fragment ions in CIMS were increased as large as those from the normal EIMS.