UDP-glucuronosyltransferase 1A9 (UGT1A9) contributes to the glucuronidation of numerous drugs. Cynomolgus monkeys are regarded as experimental animals similar to humans in studies on safety evaluation and biotransformation for drug development. In this study, the similarities and differences in the enzymatic properties of UGT1A9 between humans and cynomolgus monkeys were precisely identified. UGT1A9 cDNAs of humans (humUGT1A9) and cynomolgus monkeys (monUGT1A9) were cloned, and the corresponding proteins were heterologously expressed in Sf9 cells. The enzymatic properties of UGT1A9s were characterized by kinetic analysis of propofol glucuronidation. The amino acid homology between humUGT1A9 and monUGT1A9 was 93.2 %. Propofol glucuronidation by recombinant humUGT1A9 and monUGT1A9 exhibited substrate inhibition and monophasic Michaelis-Menten kinetics, respectively. The K m, V max and CL int values of humUGT1A9 were 15.0 μM, 1.56 nmon/min/mg protein and 107 μL/min/mg protein, respectively. The K m value of monUGT1A9 was 8.8-fold higher than humUGT1A9, and the V max and CL int values of monUGT1A9 were 15 and 2 % of humUGT1A9, respectively. These findings suggest that the enzymatic properties of UGT1A9 are considerably different between humans and cynomolgus monkeys, although humUGT1A9 and monUGT1A9 were highly conserved at the amino acid level. The information on species differences in UGT1A9 function gained in this study should help with the in vivo extrapolation of drug metabolism.