Abstract: Nobiletin is a kind of polymethoxyflavonoid with many pharmacological effects, such as antiinflammatory and antioxidation activities. This study was carried out to investigate the inhibitory effects of nobiletin on P-glycoprotein (P-gp) in vitro and in vivo. The molecular mechanism for structure-inhibition relationships of nobiletin with P-gp was investigated. Nobiletin exhibited significant inhibition (IC₅₀=2.21 μmol·L^-1) on P-gp in MDR1-MDCKⅡ cells. In the cell toxicity test, the paraquat-treated cell viability was decreased with nobiletin by inhibiting P-gp activity. In the rats PK study, the AUC_0-t of digoxin was increased 2.02 folds while the C_max of digoxin was increased 3.29 folds, when nobiletin was used in the pretreatment of SD rats. Molecular docking analysis elucidated that the formation of Pi-Pi bonds with Phe974 was the key factor for P-gp inhibition. The research findings provide important guideline for prediction of potential interaction between nobiletin and P-gp.