Abstract: A stable association complex was formed by the reaction of thiamphenicol with bromothymol blue in Tris-HCl buffer solution of pH 7. 53, leading to decrease of absorption of bromothymol blue. Linear relationship between the magnitude of decrease in absorbance at 604 nm and the mass concentration of thiamphenicol was obtained in the range within 5.3 mg·L-1, and value of apparent molar absorptivity found was 3.12×104L·mol-1·cm-1. The proposed method was used in the determination of thiamphenicol in samples of commodity thiamphenicol medicine, the results obtained was checked quite well with the labelled values, and values of recovery found by standard addition method using human urine sample as matrix were in the range of 98.3%-102%.