Abstract: The Cyclosporin in human blood was determinated by LC-MS/MS. Cyclosporin and the internal standard were extracted from blood by methanol， which was used as deproteinated solvent, and then separated on a Kromasil-C₁₈ column (20 mm×4.6 mm×5 μm). The mobile phase was consisted of methanol 1 mmol•L^-1 ammonium formate maintained at 60 ℃. The flow rate was 1.1 mL•min^-1，and 20 μL aliquot of residues were injected into the LC-MS/MS system. Detection was carried out by multiple reaction monitoring on 3200 Qtrap LC-MS/MS system. The assay is linear over the range of 0.20—2000 μg•L^-1 with a lower limit of quantitation of 0.20 μg•L^-1. Intra- and inter-day precision are less than 15%. The relative deviation is in the range of -12.60%—12.80%. The recovery of Cyclosporin is （98.1±4.7）%，and stability is good. It is a rapid, sensitive, selective and reliable method for the determination of Cyclosporin in human blood. The assay can be applied for the determination of Cyclosporin in human blood and the study on pharmacokinetics.