Abstract: To explore the anti-atherosclerotic mechanism of estrogen and especially observe the effect of estradiol on the content of cholesterol in J774a.1 mouse mononuclear/macrophage-derived foam cells which were incubated with oxidized low-density lipoproteins (ox-LDL). J774a.1 mouse mononuclear/macrophages were incubated with ox-LDL or with both ox-LDL and estradiol (1, 0.1 or 0.01 μmol·L^-1). Oil red O staining was used to observe the formation of foam cells, and cholesterol oxidase fluorometric was used to determine the content of cellular cholesterol content. Western blotting and RTFQ-PCR were used to observe the expressions of scavenger receptor class B type I (SR-BⅠ) in J774a.1 foam cells. Compared with the control cells, J774a.1 mouse mononuclear/macrophage-derived foam cells showed significantly increased contents of total cholesterol and cholesterol ester (P < 0.001) and decreased SR-BⅠ mRNA expression (P < 0.01). Estradiol treatment significantly lowered the contents of total cholesterol and cholesterol ester (P < 0.05), and increased SR-BⅠ protein and mRNA expression (P < 0.01) in the foam cells in a dose-dependent manner. Estradiol can inhibit the formation of mononuclear/macrophage-derived foam cells by decreasing the contents of total cholesterol and cholesterol ester and up-regulating the expression of SR-BⅠ in the foam cells.