Here we report a great improvement in reversal potency of cancer drug resistance when flavonoid dimers possess a functionally substituted aminopolyethylene glycol linker. The most potent compound, 18, contains a N-benzyl group at the linker. It has many advantages including (1) high potencies in reversing P-glycoprotein (P-gp) mediated resistance in LCC6MDR cells to various anticancer drugs with EC(50) in the nanomolar range, (2) low toxicity and high therapeutic index, and (3) preferential inhibition of P-gp over multidrug resistance protein 1 and breast cancer resistance protein. Compound 18 stimulates P-gp-ATPase activity by 2.7-fold and mediates a dose-dependent inhibition of doxorubicin (DOX) transport activity. Lineweaver-Burk and Dixon plots suggest that 18 is a competitive inhibitor to DOX in binding to P-gp with a K(i) of 0.28-0.34 μM and a Hill coefficient of 1.17. Moreover, the LCC6MDR cell displays about 2.1-fold lower intracellular accumulation of 18 compared to the wild type, suggesting that 18 is a P-gp substrate as well.