Treated with extremely low EuCl 3 (0 001, 0 01, 1 0 mg·L -1 ) in MS medium, the hairy root growth of Cassia obtusifolia was inhibited. The inhibition was strongest at the treatment with 0 001 mg·L -1 EuCl 3, and the dry weight of hairy roots was 22% lower than that of control. Treated with rather low EuCl 3 (10 mg·L -1 ) in Murashige and Skoog(MS) medium, the hairy root growth was enhanced, and the dry weight of hairy roots was 27% higher than that of control. In the each treatment with 0 001 to 1 0 mg·L -1 EuCl 3 in MS medium, the total content of six free anthraquinones was lower than that of control. While treated with 10 mg·L -1 EuCl 3, the total content of six free anthraquinones was 26% higher than that of control, 97% higher than the treatment of 1 0 g·L -1 EuCl 3. Treatment with 10 mg·L -1 EuCl 3 in MS medium enhances the growth of hairy root of Cassia obtusifolia and improves the total content and production of six free anthraquinones in them. Treated with extremely low EuCl 3 (0 001, 0 01, 1 0 mg · L -1) in MS medium, the hairy root growth of Cassia obtusifolia was inhibited. The inhibition was strongest at the treatment with 0 001 mg · L -1 EuCl 3, and the dry weight of hairy roots was 22% lower than that of control. Treated with rather low EuCl 3 (10 mg · L -1) in Murashige and Skoog (MS) medium, the hairy root growth was enhanced, and the dry weight of hairy roots was 27% higher than that of control. In the each treatment with 0 001 to 1 0 mg · L -1 EuCl 3 in MS medium, the total content of six free anthraquinones was lower than that of control. While treated with 10 mg · L -1 EuCl 3, the total content of six free anthraquinones was 26% higher than that of control, 97% higher than the treatment of 10 g · L -1 EuCl 3 Treatment with 10 mg · L -1 EuCl 3 in MS medium enhances the growth of hairy root of Cassia obtusifolia and improves the total content and p roduction of six free anthraquinones in them.