A simple, rapid and sensitive colorimetric method for the determination of captopril is presented in the present paper. It is based on the fact that captopril can induce the aggregation of AgNPs, thereby resulting in their yellow-to-red color change and the absorbance decrease at λ395 nm. The mechanism of the aggregation effect was discussed in detail. Under the optimized conditions, the linear range of determination of captopril was 1~35 μg·mL-1 with correction coefficient 0.998 4. The detection limit of the method for captopril was 0.7 μg·mL-1. The method has been applied to the determination of captopril in tablets with satisfactory result.