A new flavonoid, (2R,3R)-3,7,4'-trihydroxy-3',5'-dimethoxyflavanone, named sophorayunnanol (1), together with eight known polyphenols (2-9), were isolated from the roots of Sophora yunnanensis C.Y.MA (Leguminosae). The structure including absolute stereochemistry of 1 was determined by spectroscopic (high resolution (HR)-MS, 1D- and 2D-NMR, and circular dichroism (CD)) methods. The inhibitory activity of these compounds was examined against nitric oxide (NO) production by lipopolysaccharide (LPS) and interferon (IFN)-γ activated macrophages, RAW264.7 cells. 3,7,3',5'-Tetrahydroxy-4'-methoxyflavone (6) and piceatannol (9) showed potent inhibitory activity against the production of NO with IC(50) values of 14.4±2.5 and 12.9±1.8 µM, respectively. This article is the first report on phytochemical study of S. yunnanensis.