Estimating the extent to which drugs inhibit uridine 5′-diphosphate-glucuronosyltransferases1A1 (UGT1A1) enzyme activity is important for predicting hepatotoxicity and neurotoxicity. UGT1A1 enzyme activity is commonly evaluated by detecting the elimination of bilirubin substrate or the generation of bilirubin glucuronides. However, the present methods are inadequate for accurately assessing bilirubin metabolism, selecting incubation conditions, and comparing different systems. Therefore, in our study, we first established a ultra-performance liquid chromatography (UPLC)-MS/MS method to identify bilirubin and bilirubin glucuronides. To ensure the reaction was linear, we performed assays to optimize the protein concentration and incubation time. Finally, we measured UGT1A1 activity using three different systems. Data revealed the optimum incubation conditions were 10 min with 0.5 mg/mL human liver microsomes (HLM), recombinant human UGT1A1 (rUGT1A1), and rat liver microsomes (RLM). Bilirubin glucuronidation obeyed Michaelis-Menten kinetics in all three systems. The diversity in bilirubin metabolism among species was revealed. rUGT1A1 had the strongest binding affinity for bilirubin, but the lowest metabolism velocity. Compared with the other systems, RLM exhibited a significant difference. It has the lowest CL int and the highest K m . The difference in parameters between three systems may be attributable to the species differences. In conclusion, these in vitro studies provide useful information regarding drug interactions and the prediction of toxicity for future studies.