Manassantin B, a dilignan isolated from Saururus chinensis, significantly inhibited proliferation in HSC-T6 cells in concentration- and time-dependent manners. In addition, treatment of HSC-T6 cells with manassantin B changed cell morphology from flattened myofibroblastic membranous morphology, representing activation state, to slender shape, representing quiescent state. Furthermore, manassantin B effectively reduced collagen content in HSC-T6 cells. These results suggested that manassantin B exerted antifibrotic activity in HSCT6 cells, in part, via inhibition of cell proliferation and decrease of collagen production.