Three new coumarins containing a C10 terpenoid side chain, clauslactones R—T (1—3), together with 14 known coumarins (4—17) and 11 known carbazole alkaloids (18—28), were isolated from the leaves and stems of Clausena excavata. Their structures were established by detailed spectroscopic analyses. Furthermore, the stereochemistry of 1 was confirmed by single-crystal X-ray diffraction analysis, which was the first example among coumarins with a C10 terpenoid side chain. Additionally, compounds 22 and 27 were found to show moderate topoisomerase II inhibitory effects at 50mM.