Two new phenolic compounds, 6-hydroxy-7-methoxy-3-(4-methoxyphenyl)-2 H -chromen-2-one ( 1 ) and 7-hydroxy-6-methoxy-3-(4-methoxyphenyl)-2 H -chromen-2-one ( 2 ), together with six known phenols ( 3–8 ), were isolated from the stems of Cassia siamea . The structure of 1–8 was elucidated by spectroscopic methods including extensive 1D and 2D NMR techniques. Compounds 1–8 were evaluated for their anti-tobacco mosaic virus (anti-TMV) activity. The results showed that compounds 1 and 2 have potential anti-TMV activity with inhibition rates of 28.2 and 23.5%, respectively. Compounds 3–8 also showed anti-TMV activity with inhibition rates in the range of 11.6–19.8%.