Abstract: The adduct and fragmental behavior of ivermectin was studied in both positive and negative ion detection mode by using electrospray ionization time of flight mass spectrometry.The optimal organic solvent content for positive ion was 50∶50 methanol/water.The maximal intensity of M+NH 4 + was achieved when nozzle potential was 120V,while the peaks ofM+Na + and M+K + were highest at 300V.No peak of M+H + was observed in the positive ion detection mode of ivermectin.In negative ion detection mode,90∶10 methanol/water was the proper solvent.At lower nozzle potential,the high intensity of adduct ions were formed,which are M+Cl -,M+HCOO -,M+CH 3COO -and M+C3H5O3 -.With the increasing nozzle potential,the intensity of fragmental ions were increased,which corresponded to the loss of H,the lose of one or two H 2O,sugar moieties of Diels Alder fragment ions from M-H -.When nozzle potential was 200V,the highest intensity of M-H - was achieved.