Rhus verniciflua Stokes (RVS), used as a food additive and a traditional herbal medicine, has both antioxidant and antitumor activities which are known to be closely associated with the polyphenolic compounds that it contains. In the present study, we purified a fraction from a crude acetone extract of RVS, named RCMF (RVS chloroform-methanol fraction), and evaluated its ability to scavenge free radicals and inhibit cell growth. In addition, the active compounds responsible for the activities were identified. Results showed that RCMF contained an antioxidant potential and strongly suppressed the proliferative capability of B lymphoma cells. RCMF-mediated suppression of cell growth was verified to be apoptotic, based on the increased DNA fragmentation and low fluorescence intensity in the nuclei after propidium iodide staining, and also on the appearance of DNA laddering. Finally, EI-MS, 1H-NMR, and 13C-NMR spectra confirmed that RCMF contained flavonoid derivatives, including protocatechuic acid, fustin, fisetin, sulfuretin, and butein, suggesting that these flavonoid derivatives are the main active compounds responsible for the antioxidant and antiproliferative activities of RCMF.