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Bioactive-guided phytochemical investigation of <i>Euphorbia antiquorum</i> L. growing in Vietnam led to the isolation of five <i>ent</i>-atisanes, one <i>seco</i>-<i>ent</i>-atisane, and one lathyrane (ingol-type). The structures were elucidated as <i>ent</i>-1<i>α,</i>3<i>α,</i>16<i>β,</i>17-tetrahydroxyatisane (<b>1</b>), ethyl <i>ent</i>-3,4-<i>seco</i>-4,16<i>β</i>,17-trihydroxyatisane-3-carboxylate (<b>2</b>), <i>ent</i>-atisane-3-oxo-16<i>β</i>,17-acetonide (<b>3</b>), <i>ent</i>-3<i>α</i>-acetoxy-16<i>β</i>,17-dihydroxyatisane (<b>4</b>), <i>ent</i>-16<i>β</i>,17-dihydroxyatisane-3-one (<b>5</b>), calliterpenone (<b>6</b>), and ingol 12-acetate (<b>7</b>). Their chemical structures were unambiguously determined by analysis of one-dimensional (1D) and two-dimensional (2D) nuclear magnetic resonance (NMR) and high resolution mass spectrometry, as well as by comparison with literature data. Among them, <b>1</b> is a new compound while <b>2</b> is an ethylated artifact of <i>ent</i>-3,4-<i>seco</i>-4,16<i>β</i>,17-trihydroxyatisane-3-carboxylic acid, a new compound. Isolates were evaluated for alpha-glucosidase inhibition. Compound <b>3</b> showed the most significant inhibitory activity against alpha-glucosidase with an IC₅₀ value of 69.62 µM. Further study on mechanism underlying yeast alpha-glucosidase inhibition indicated that <b>3</b> could retard the enzyme function by noncompetitive.