Abstract: OBJECTIVE To set up a practical RP-HPLC method to determine the concentration of cyclosporin A (CsA )in the whole blood. METHODS CsA was extracted from whole blood with diethyl and removed the endogenous interference in blood with n-hexane ether and separated using Zorbax SB-C₁₈ column(4.6 mm×150 mm, 3.5 μm). Mobile phase was acetonitrile-methanol-deionized water-isopropyl(60∶10∶29∶1). The flow rate was 1.5 mL·min^-1. The temperature of column was set at 70 ℃. The detecting wavelength was 210 nm. The internal standard was cyclosporine D. RESULTS A good linear relationship was obtained between the concentration of CsA from 25 ng·mL^-1 to 1 600 ng·mL^-1(r=0.999 8). The average method recovery rate of CsA was 99.51%. The intra- and inter-day RSD were less than 5%. The detection limit of CsA was 5 ng·mL^-1. CONCLUSION The established method was simple, accurate, sensitive, reproducible and large-scale of detection range and suitable for monitoring blood-drug concentration of CsA .