Arteannoide A (1), an unusual cadinane dimer featuring a rare fused 6,8-dioxabicyclo[3.2.l]octan-7-one ring system, arteannoides B and C (2 and 3), two novel heterodimers incorporating a rearranged cadinene sesquiterpenoid and a phenylpropanoid, together with two new rearranged cadinene sesquiterpenoids 4 and 5, were isolated from Artemisia annua L. Their structures were determined by a combination of NMR spectroscopy, electronic circular dichroism calculations, and X-ray diffraction analyses. Compounds 2 and 3 showed inhibition of nitric oxide production in lipopolysaccharide-induced RAW 264.7 mouse macrophage cell lines with IC50 values of 4.5 and 2.9 μM, respectively.