Seventeen new lathyrane diterpenoids (1-17) and two known analogues have been isolated from an ethanolic extract of Euphorbia micractina roots. Their structures including absolute configurations were determined by spectroscopic data interpretation and single-crystal X-ray crystallography. Compound 10 showed activity against HIV-1 replication in vitro, with an IC50 value of 8.2 μM. Compounds 6, 7, 11, 14, 15, and 18, at 10(-6) M, showed significant vascular-relaxing activities against phenylephrine-induced vasoconstriction with relaxation rates of 48%, 41%, 42%, 48%, 50%, and 53%, respectively.