Abstract: Currently,malignancy is one of the serious threats to human health and life.In China's urban population,tumor-induced deaths account for 25% of all diseases.Clinical treatment of malignant tumors commonly used three kinds of methods of surgical treatment,radiotherapy and chemotherapy,chemotherapy play an irreplaceable role in the treatment of malignant tumors.Some patients after the effective chemotherapy will continue to relapse,the most important reason is that the formation of multidrug resistance (MDR) to limit the efficacy of chemotherapy.Clinical application of combined chemotherapy can play the largest cytotoxic effect,but also because of the production of MDR failed.Therefore,overcome the multidrug resistance of tumors is an key to future research on tumor therapy.The mechanism of MDR is a very complicated process,influenced by many factors.Tumor cells cause multidrug resistance in a variety of ways,including drug-related proteins,DNA damage and repair dysfunction,cell autophagy,reduced drug accumulation and increased drug output,metabolic detoxification,drug targets and signal transduction molecules changes and other mechanisms.Tumor multidrug resistance is associated with P-glycoprotein,breast cancer resistance protein,multidrug resistance-associated protein,lung resistance protein,topoisomerase and apoptosis-related proteins.There are many reversal agent used in clinical,but the effective and toxic side effects of small MDR reversal agent remains to be further studied.Therefore,to study the mechanism of MDR and tumor cells produce resistance to drugs will be helpful to develop more effective antitumor drugs.This paper presents a review on the current research of molecular mechanism of MDR.