Abstract: Based on the reduction of Fe(Ⅲ) by tiopronin ［with its chemical name of N-(2-mercaptopropionyl)-glycine］ to Fe(Ⅱ), and the color reaction of the Fe(Ⅱ) with o-phenanthroline to give a complex having its absorption maximum at 508 nm, as well as on the application of flow-injection technique, an indirect FI-photometric method for determination of tiopronin was proposed. A FI analyzer with grating spectrophotometer as detection unit was designed and fabricated. A frequency of 85 determinations per hour was attained. Linear relationship between values of absorbance and mass concentration of tiopronin was obtained in the range of 4.00~450 mg·L-1, and the detection limit (3s/k) found was 0.56 mg·L-1. The proposed method was used for determination of tiopronin in drugs of tablets and injections, the results obtained were in consistency with the results found by the standard method. Recovery was tested by standard addition method giving values av. recovery of 100.5% (tablet sample) and 99.2% (injection sample).