Persicaria thunbergii has been utilized for the treatment of cancer as a folk medicine. We examined the effect of isorhamnetin, a flavonoid isolated from Persicaria thunbergii, on angiogenesis in vitro and in vivo. Basic fibroblast growth factor (bFGF) is a potent angiogenic factor found in various tumors. In this study, we found that the isorhamnetin decreased bFGF-induced human umbilical vein endothelial cells (HUVECs) proliferation and migration in a concentration-dependent manner (5, 10 and 20 μM) whereas, it did not inhibit bFGF-induced capillary-like formation of HUVECs. The chicken chorioallantoic membrane assay revealed that addition of isorhamnetin (10, 20 and 40 μM) displayed an antiangiogenic effect in vivo. These results suggest that the isorhamnetin inhibits the proliferation and migration of endothelial cells induced by bFGF, which may explain its anti-angiogenic properties.