Four novel chemical constituents, cannabiside B, cannabiside C, cannabiloid B, and cannabilactone A, have been isolated from the n-butanol extract of the herb of Senecio cannabifolius Less. (Compositae). The structures of the isolated compounds were established as 3-[3,4-dihydroxy-5-(hydroxymethyl)tetrahydrofuran-2-yl]- 5-hydroxyfuran-2(5H)-one (1), 8,10-dihydroxy-7-(hydroxymethyl)-4-methyl-1, 6-dioxaspiro [4.5] dec-3-en-2-one (2), 6-(1,2-dihydroxy-ethyl)-3-[3-(3,4-dihydroxy-5-hydroxymethyl-tetrahydro-furan-2-yl)-5-hydroxy-pyrrol-2 ylidene]-4-hydroxy-tetrahydro-pyran-2-one (3), and 5 hydroxy-3-{[5-(hydroxylmethyl)furan-2-yl] methylene} benzofuran-2(3H)-one (4), respectively, by spectroscopic (UV, IR, NMR, and MS) methods. All the compounds showed antibacterial activities against Staphylococcus aureus and Bacillus subtilis but not Escherichia coli.