Effects of tea plyphenols on angiotensinⅠconverting enzyme（ACE） activity were studied in this paper. Activity of ACE were determined by in vivo and in vitro on rats. The concentrationof polyphenols were 0, 0.31, 0.62, 1.24, 2.48, 4.96βmmol/L in vitro and polyphenols concentrations were 0, 45, 90, 180βmg/kg in vivo. ACE activity decreased markedly with the increase of tea polyphenols level in vivo and in vitro test. Results showed that activity of ACE is inhibited by tea polyphenols in vivo and in vitro test and these results showed that tea polyphenols possess the function of reducing blood pressure possibly.