Seven compounds were isolated from the n-BuOH soluble fraction of stem of Rhododendron mucronulatum as cytotoxic principles against brine shrimp lethality test. On the basis of spectral data, seven compounds were identified as (-)-catechin (1), (+)-epicatechin (2), taxifolin (3), scopoletin (4), quercetin (5), $taxifolin-3-O-{alpha}-L- arabinopyranoside$ (6), and astragalin (7). Among tested compounds, $taxifolin-3-O-{alpha}-L-arabinopyranoside$ (6) exhibited potent activity with $LC_{50}$ value at 4.6 ${mu}g/ml$.