Two new anthraquinones, siameaquinones A ( 1) and B ( 2), together with seven known anthraquinones ( 3-9), were isolated from the stems of Cassia siamea. The structure of 1-9 was elucidated by spectroscopic methods, including extensive D and D NMR techniques. Compounds 1 and 2 were also evaluated for their cytotoxicities against five human tumor cell lines. The results revealed that compound 1 showed moderate cytotoxicities against A549 and SHSY5Y cells with IC values of 5.2 and 6.3 μM, and compound 2 showed moderate cytotoxicities against NB4 and SHSY5Y cells with IC values of 8.8 and 7.5 μM, respectively. [ABSTRACT FROM AUTHOR]