Abstract: This paper studied the pattern of the Leukamenin E, an entkaurene diterpenoid regulation on the seed germination, hypocotyl elongation, and root growth and development in Arabidopsis thaliana. In addition, this work also discussed the mechanism of primary root growth, lateral root and root hair development regulated by Leukamenin E possibly through the phytohormone auxin and ethylene pathway in A. thaliana. The results showed that within 10-160 μmol·L^-1, Leukamenin E had no obvious effect on seed germination rate; however, higher levels of Leukamenin E (80-160 μmol·L^-1) significantly reduced the seed germination speed. The inhibiting effect of Leukamenin E on root growth was more obvious than on hypocotyl elongation. Furthermore, Leukamenin E inhibited the primary root growth through reducing cell mitosis and elongation. Leukamenin E accelerated the lateral root initiation and affected the lateral root numbers, and also reduced the root hair density and shortened the root hair length. Promoting root hair growth was stimulated by exogenous ethephon, an ethylenereleasing agent, and could be reversed by Leukamenin E treatment, which is similar to the effect of the Ag⁺, an ethylene action inhibitor. These results strongly suggested that the inhibition of Leukamenin E on root hair growth may depend on the ethylene pathway. Leukamenin E markedly increased the IAA level in root, which was detected by using the flow injection chemiluminescence method and the enzymelinked immunosorbent assays, indicating that auxin may play an important role in regulation of root growth by Leukamenin E in A. thaliana.