For most cancer, the treatment of choice is still being chemotherapy despite of its severe adverse effects, systemic toxicity and limited efficacy due to the development of multidrug resistance (MDR). MDR leads to chemotherapy failure frequently due to an overexpression of ABC transporters such as P-glycoprotein (P-gp/MDR1/ABCB1) that limit the efficacy of chemotherapeutic drugs. [1]P-gp is a transmembrane protein that is encoded by the MDR1 gene and is the best studied drug efflux pump of the family of ABC transporters. A wide range of therapeutics drugs are substrates of p-gp, including a variety of anticancer agents. [2] Therefore, exploring effective P-gp inhibitors has become a major way to improve MDR in anticancer therapy. To date, three generations of P-gp inhibitors have been developed; however, none of them could achieve beneficial outcomes. As a result, scientist turned to search for p-gp inhibitors from natural compounds,[3, 4] which are generally considered to be safer and nontoxic at usual doses. Among these natural products, caffeic acid is considered to be a potential compound. Caffeic acid has shown biological activity as an antioxidant. Moreover, caffeic acid phenethyl ester(CAPE) extracted from propolis also possess a wider biological activities including antioxidant, anti-inflammantory, antiviral, antiviral and immunomodulatory effects. [5] However, the mechanism is still unclear. Thus, our study group will explore its possible action mechanism with caffeic acid on p-gp. Caffeic acid may be a future lead compound to reverse the effect of MDR by focusing on the inhibition of ABC transporters against MDR cells and restoration of sensitivity toward chemotherapeutic drugs.The aim of this study is to identify whether caffeic acid is a p-gp inhibitor and what the molecular mechanisms are. Additionally, the numerous single nucleotide polymorphisms(SNPs) on ABCB1 gene of caffeic acid will give rise to p-gp inhibition or even reversing MDR in cells.