Objective Epidemiological studies have suggested that alcohol drinking is closely associated with diverse tumor development,but fewer laboratory studies or mechanism analysis is made.Our study aims to explore the effect of alcohol on the tumorigenicity and metastasis of breast cancer in nude mice in vivo and on the malignant behavior in vitro, and whether it is related to endoplasmic reticulum stress (ERS).Methods We first established the nude mouse model of breast cancer with chronic alcohol consumption (2%) and to detect the effects of ethanol on tumor growth and metastasis.We also observed the changes of malignant biological behavior of EO771 cells in cell culture induced by chronic alcohol treatment (0.2%) using MTT,Transwell and soft agar tests,and detect the expression of ROS and ERS marker,i.e.p-eIF2a,Bip,and XBP1-s.Results The in vivo experiment showed that alcohol drinking enhanced the growth and metastases of breast cancer in nude mice.The in vitro test showed that chronic exposure to ethanol also promoted the cell proliferation and migration ability,increased the ROS expression,activated ERS pathway,and enhanced the expression of p-eIF2a,Bip and XBP1-s.Conclusions Alcohol drinking may promote the malignant behavior of breast cancer cells through endoplasmic reticulum stress.