In the Arabidopsis genome, approximately 120 UDP-glycosyltransferases (UGTs), which are classified to 14 distinct groups (A to N), have been annotated. In general, UGTs catalyze the transfer of sugars to various acceptor molecules including flavonoids. To provide better understanding of the physiological roles of UGTs, we have attempted to analyze several putative flavonoid UGTs by a combination of molecular genetic and metabolomic approaches. Thus far, our analysis reveals that several loss-of-function mutantions in flavonoid UGTs display improved resistance to methyl viologen (paraquat), which was thought to cause oxidative stress. Our results suggest that the Arabidopsis flavonoid UGTs play an important role in glycosylation pathways in response to oxidative stress.