A simple and selective method to determine gatifloxacin using an optical flow-through sensor has been developed. The optical sensor is based on the emission intensity from the Tb(III) solution sensitized by gatifloxacin. The excitation of gatifloxacin was occurred by the chemiluminescence from the reaction of KMnO4 and Na2SO4 solutions. Under the optimal conditions, the emission intensity increased linearly with increasing gatifloxacin concentration from 1.1 X 10^-8 to 1.6 X 10^-6 M and the detection limit (3σ) was 1.0 X 10^-8M.