Abstract: The A. O. A. C. official method for assaying pyrethrin is in wide use as it has been considered to be reliable and accurate. However, the precedure is rather tedious and time-consuming. The authors propose to modify the method, by (1) using a modified extraction device, the sample can be extracted completely within one hour with petroleum ether, and (2) directly titrating chrysanthemum monocarboxylic acid and chrysanthemum dicarboxylic acid, subsequently extracted with petroleum ether and ether respectively, in these organic solvents with potassium methoxide solution using bromothymol blue as indicator. Two samples of pyrethrum powder and five samples of pyrethrum extract preparations were determined, giving results agreed well with those obtained by A. O. A. C. official method.