Natural compounds possessing antioxidant effect are intensively seeking for prevention and cure of many diseases including cancer. A cell-base antioxidant test is a more biological relevant method than chemistry antioxidant activity assays. In this study, the authors established a protocol using DCFH-DA (2',7'-dichlorodihydrotluorescein diacetate) probe to detect antioxidant activity in Hep G2 cell line. After taken up by Hep G2 cells, DCFH-DA is trapped within cells and oxidized to a tlourescent dichlorotluorescein (DCF) byintracellular reactive oxygen species (ROS). Antioxidant activity of compounds is measured through ability of compound to reduce ROS. Some parameters of the protocol were determined including cell seeding density, DCFH-DA and AAPH concentrations. Cell density of 6 x 10 exponent 4 cells/well, concentration of DCFH-DA (40 uM) and AAPH (600 uM) showed the best results. Utilized to determine intracellular antioxidant effect of Quercetin, the protocol showed high reproducibility, apt to be used for antioxidant screening of natural compounds. The traditional remedies Hoang Lien Giai Doc Thang, at a concentration of 10 percent, reduced intracellular ROS on Hep G2 cells.