Water extract of Astragali radix (ARW) was prepared and assessed for chemopreventive activity against breast cancer cells by measuring the effect on proliferation of human breast cancer cells and cytochrome P450 (CYP) 1A1, CYP 2E1, and cyclooxygenases-2 (COX-2) activities. ARW inhibited the proliferation of estrogen-independent MDA-MB-231 human breast cancer cells. There were 29.4% and 35.9% inhibition of CYP 1A1 activity with the treatment of 100 and 200 μg/mL ARW, respectively. ARW also inhibited CYP 2E1 activity. COX-2 activity was significantly inhibited with the treatment of 100 (p<0.05), 200 (p<0.01) and 300 (p<0.005) μg/mL ARW. These results suggest that Astragali radix may have breast cancer chemopreventive activity by inhibiting the growth of human breast cancer cell and CYP 1A1, CYP 2E1, and COX-2 activities.