Two new alkaloids, 6-(hydroxymethyl)-3,9-dimethyl-7H-benzo[de]- quinolin-7-one (1) and 6-(hydroxymethyl)-8-methoxy-3,9-dimethyl-7H-benzo[de]- quinolin-7-one (2), together with four known alkaloids (3-6) were isolated from the seeds of Cassia alata. Their structures were determined by means of HRESIMS and extensive 1D and 2D NMR spectroscopic studies. Compounds 1-6 were tested for their anti- Tobacco mosaic virus (TMV) activities, and the results showed that compound 2 exhibited high anti-TMV activity with inhibition rates of 38.5%. This rate is higher than that of the positive control. The cytotoxicities of compounds 1-6 against five human tumor cell lines (NB4, A549, SHSY5Y, PC3 and MCF7) were also tested. Compounds 1-6 showed weak inhibitory activities against some tested human tumor cell lines with IC50 values in the range of 2.5-7.5 μM. © 2016 The Japan Institute of Heterocyclic Chemistry.