Activity-guided fractionation and repeated column chromatography afforded two cytotoxic compounds R-3 and R-4 from the root of Rumex japonicus HOUTT. Compou nds were identified as musizin and emodin, respectively, by the physicochemical and spectral data. Besides R-3 and R-4, two compounds R-1 and R-2, chrysophanol and physcion, respectively, were also isolated. The compound R-3 and R-4 exhibited cytotoxicity against cultured human tumor cell lines, A-549, SK-OV-3, SK-MEL-2, XF498 and HCT15 with $ED_{50}$ values ranging from 2.68 to $10.06{mu}g/ml$.