A new alkaloid 8-methoxy-2,5-dimethylpyrano[2,3,4-ij]isoquinoline (1), was isolated from the stem of Cassia siamea. Its structure was elucidated by spectroscopic methods, including extensive 1D- and 2D- NMR techniques. Compound 1 was also tested for its anti-tobacco mosaic virus activity and it showed high anti-tobacco mosaic virus activity with inhibition rate of 33.5 %.