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 $20;{mu}M$) 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 $40;{mu}M$) 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.