Abstract: Dilute ferric chloride solution has long been used as a color reagent for the identification of santonin. The present work makes use of this reagent for its quantitative determination. Factors which may affect the color intensity have been studied and a procedure is proposed as follows: A definite volume of alcoholic extract containing less than 0.25mg.of santonin in a test-tube is evaporated to dryness in a sand bath at about 100℃.After cooling, 3ml.of H₂SO₄ (1:1) and 0.1 ml.of 0.8% FeCl₃ solution are added.Heat the test- tube in a boiling water bath for 10 minutes,cool with tap water and add 3 ml.of 95% alcohol.Mix well and transfer to an absorption cell fitted with a stopper. Read the transmission in an electrophotometer within 35 minutes and calculate the result from a standard curve.Beer's law is obeyed within the range of 0.025- 0.25mg.The accuracy is ±4%. The above procedure has been used to analyza 5 samples of Artemisia incana Keller and two samples of tablets of santonin and santonin and calomel. The results are in good agreement with those obtained by classical methods. The method is simple and rapid,the analysis of a crude sample or santonin tablets being completed within 1 day.