To investigate the chemical constituents and pharmaceutical value of Euphorbia atoto,18 compounds were isolated from the ethanol extract by silica gel,Rp-18 and Sephadex LH-20 gel column chromatographies.With comprehensive analysis of the NMR and MS data,their structures were identified as (+)-6-oxocinnamolide (1),6β-hydroxycinnamolide (2),kaempferol (3),6,9-epoxy-ergosta-7,22-dien-3-ol (4),4α,4β,14α-trimethyl-9β,19-cyliopregnane-3,20-dione (5),sikkimenoid F (6),(24R)-3β-3-(acetyloxy) eupha-7,25-dien-24-ol (7),olibanumol J (8),myricarin B (9),aurantiamide acetate (10),viscumamide (11),N-benzoylphenylalaninyl-N-benzoylphenylalaninate (12),scopoletin (13),corchoionol C (14),indole-3-carboxylic acid methyl ester (15),3ξ-(1ξ-hydroxyethyl)-7-hydroxy-1-isobenzofuranone (16),p-hydroxybenzaldehyde (17) and 9,10- dihydroxyoctadecanoic acid (18).Eighteen compounds were isolated from E. atoto for the first time,except compounds 10,13,14 and 17 other compounds were isolated from Euphorbia for the first time too.Compound 1 was a new natural product.Compound 1 was a new natural product.Activity screening showed that 2 inhibited LPS-induced NO production in RAW 264.7 macrophages with IC₅₀ at 24.81±1.58 μmol/L (IC₅₀ of positive control quercetin is 15.23±0.65 μmol/L).