Nonsteroidal anti-inflammatory drugs (NSAIDs) can cause idiosyncratic liver injury. Mechanisms involved in NSAID-induced liver injury are complex. Previous studies have suggested that acyl glucuronide of NSAIDs (NSAIDs-Glu) plays an important role in the development of liver injury via covalently binds to proteins and the resultant adduct induces immunological toxicity. As only some NSAIDs-Glu are commercially available, the evaluation of covalent protein adduct formation using ready-made NSAIDs-Glu is difficult and inconvenient. Moreover, glucuronidation potency varies with the NSAID, including stereoisomers. Therefore, in this study, we simultaneously examined the glucuronidation and covalent adduct formation using enantiomers of parent NSAIDs (ibuprofen, naproxen, pranoprofen, ketoprofen, and flurbiprofen) in rat liver microsomes. Glucuronides and covalent adducts were quantified by HPLC. The amount of covalent adduct increased with NSAIDs-Glu formation in the rat liver microsomes in a time-dependent manner. A significant positive correlation was observed between the AUC of NSAIDs-Glu and that of covalent adduct, except ketoprofen. Although ketoprofen exhibited the highest glucuronidation rate among the NSAIDs investigated, the amount of covalent adduct was similar to that for pranoprofen, which had the lowest glucuronidation rate. Thus, it may be difficult for ketoprofen glucuronide to covalently bind with proteins in the rat liver microsomes. Our results suggested that the amount of glucuronide formed is a key factor in predicting covalent bond formation with protein in NSAIDs, in addition to degradability and bindability with proteins of NSAIDs-Glu. Further studies are required to confirm the relationship between the tendency of glucuronidation and the formation of covalent adducts of NSAIDs.