This study was carried out to investigate the effect of enzyme treatment with $eta$-glucosidase on antioxidant capacity of the mulberry leaf extract (MLE) using oxygen radical absorbance capacity (ORAC) and cellular antioxidant capacity (CAC) assay. The MLE was prepared by autoclaving at $121^{circ}C$ for 15 min and treated with $eta$-glucosidase for 9 hr. High pressure liquid chromatography (HPLC) analysis showed that only qercetin-3-$eta$-D-glucose (QT-G) among quercetin (QT) glycosides of MLE, including QT-G, quercetin-3-O-glucose-6"-acetate (QT-GA), and rutin (RT), was converted into QT by 3 hr treatment with $eta$-glucosidase. The in vitro peroxyl radical- and hydroxyl radical scavenging capacity significantly increased after the enzyme treatment using $eta$-glucosidase for 6 and 9 hr, respectively. The metal chelating activity increased after the enzyme treatment using $eta$-glucosidase for 3 hr. The intracellular antioxidant capacity of MLE to protect AAPH- and $Cu^{2+}$-induced oxidative stress in HepG2 cells significantly increased after the enzyme treatment using $eta$-glucosidase for 3 and 6 hr, respectively, indicating that QT may be released from QT-G by $eta$-glucosidase and penetrate into cell membrane so that it can contribute to the intracellular antioxidant capacity of MLE.