AIM To investigate the mechanism of action of Methimazole in inhibiting iodination of tyrosyl residues in bovine serum albumin. METHODS ①Purified the thyroid peroxidase from the thyroid specimen removed from patient with Graves disease and established the TPO urea hydrogen peroxide(UHP) catalysed tyrosine(in BSA) iodination system.② Incubated rough TPO(rTPO) or dialyzed TPO(dTPO) with methimazole and/or potassium iodide in 37℃ for 12 hours,then dialyzed against 0 1 mmol·L -1 PB, and determined the absorbance at 700 nm after adding hydrogen peroxide and 3,3,5,5 tetramethylbenzidine(TMB). ③Increased the concentration of UHP in BSA tyrosine iodination system and measured the radioactivity of iodinated tyrosine. RESULTS TPO activity had no significant deference between rTPO+PB, rTPO+methimazole and dTPO+methimazole +potassium iodide+hydrogen peroxide. While added hydrogen peroxide into dTPO(with or without addition of methimazole), the TPO activity decreased sharply( P <0 01)except those which potassium iodide were added into. In BSA tyrosine iodination system the methimazoles inhibition was reduced by the increment of the concentration of UHP. CONCLUSION methimazole consume the active oxygen that is required for tyrosine iodination,which consequently results in the decrement of thyroxine synthesis, and has no direct inhibition on TPO activity.