Quercetin has been suggested to exert its pharmacological effects, at least in part, via its metabolites, such as glucuronides. Quantitative regioselectivity analyses are important to understand the contributions of UDP-glucuronosyltransferases (UGTs) to the pharmacological activity of quercetin. The present work obtained active UGT1A9 and UGT1A3 enzymes with a Bac-to-Bac expression system, and quercetin was metabolized by each of them to four monoglucuronides (7-, 3-, 4'- and 3'-glucuronide). Enzymatic kinetic parameters of each glucuronide were calculated to elucidate quantitatively UGT1A9's and UGT1A3's regioselectivities for quercetin. UGT1A3's highest glucuronidation efficiency was observed for the 3'-glucuronide, then the 3-, 4'- and 7-glucuronide. The catalytic efficiency order for UGT1A9 was 3->7->3'->4'-glucuronide.