A fiber-optic evanescent wave biosensor has been developed based on the principle of evanescent wave while light propagates in optical fiber. Sensitivity limit of 0.01 nmol/l is obtained from the detection of Cy5 serial solutions with various concentrations. And an ideal result of signal-to-noise ratio of 4.61 is obtained when the biosensor is used to measure Legionella pneumophila solution of 0.01 μmol/l. All the results are comparable with that obtained by a commercial biochip scanner.