Angiotensin-I-converting enzyme (ACE) is an important enzyme in the regulation of blood pressure. In recent years, several ACE inhibitory peptides had been isolated from the enzymatic digests of milk proteins, or fermented milk prepared with lactic acid bacteria. In this study, to analyze the inhibition mechanism of ACE by Lactobacillus casei 1.2435-fermented milk, the method of Cushman-Cheung was used to examine the change of ACE activity before and after being treated with the supernatant of fermented milk. In addition, the promoter of human ACE was also cloned and inserted into pGL3 luciferase reporter plasmid, and then the effect of the supernatant of fermented milk on the transcription of ACE was detected by luciferase reporter assay in COS-7 cells. The results showed that the ACE inhibitory rate of L. casei 1.2435-fermented milk was about 97.6%, and it would decrease to 73.6ÿ and 75.7ÿ after being treated with chloroform and boil, respectively. Besides, it seemed that the fermented milk and the lysis product of the bacteria also had an influence on the activity of ACE promoter.