Gochujang is a red pepper-soybean paste and a popular Korean fermented food. The aim of this study was to investigate the behavior of flavonoids and activity of ${eta}$-glucosidase during fermentation of gochujang prepared using Aspergillus oryzae. The content of flavonoid glycosides (genistin, daidzin, apigenin 7-O-${eta}$-$small{D}$-glucopyranoside, and quercetin 3-O-${alpha}$-$small{L}$-rhamnopyranoside) and their aglycones did not change significantly during preparation of gochujang. However, ${eta}$-glucosidase was produced by A. oryzae. The polar layer of the gochujang methanol (MeOH) extract more strongly inhibited the ${eta}$-glucosidase activity of the crude enzyme extracted from gochujang than that of the nonpolar layer. The red pepper extract and high salt solution of the gochujang ingredients showed high ${eta}$-glucosidase inhibition activity. These results indicate that flavonoid glycosides were not hydrolyzed by the ${eta}$-glucosidase produced by A. oryzae during gochujang fermentation, which may have been due to the salt and hydrophilic compound(s) present in red pepper as main ingredients of gochujang.