From the n-BuOH soluble traction of the 70% aqueous acetone extract of Rhododendron mucronulatum stem, twelve compounds were isolated. On the basis of spectral data, they were identified as scopoletin (1), (+)-taxifolin (2), quercetin (3), (-)-catechin (4), (+)-epicatechin (5), scopolin (6), lyoniside (7), ssioriside (8), fraxin (9), $(+)-lyoniresinol-3{alpha}-O-{eta}-D-glucopyranoside$ (10), $(+)-taxifolin-3-O-{alpha}-L-arabinopyranoside$ (11), and astragalin (12), respectively. All isolated compounds were tested antioxidant activity against 1, 1-diphenyl-2-picrylhydrazyl (DPPH) radical. Compounds 2 and 3 showed the potent antioxidant activity, and compounds 5, 8, and 11 showed moderate activity.