Abstract: External stimuli such as hypoxia or nutritional deprivation may lead to endoplasmic reticulum (ER) stress which is closely related to the survival of cancer cells. In this study, we revealed that TRIM25, as a novel inducible protein during ER stress and its role in tumor cells, which provides evidences for new tumor targets. In this research, we constructed TRIM25 stable knockdown cell line in MCF7 and detected the effects of TRIM25 knockdown on ER stress, unfolded protein reaction (UPR) signaling pathway and ER stress induced apoptosis. Then, we detected expression of TRIM25 in different breast cells and analyzed the correlation between TRIM25 and prognosis of breast cancer patients by using bioinformatics. Our results identified that ER stress significantly induce the expression of TRIM25. Moreover, TRIM25 knockdown promotes the apoptosis of MCF7 cells through inducing ER stress and activating unfolded protein reaction signaling pathway. In addition, bioinformatics analysis shows that the expression of TRIM25 is up-regulated during the transition from primary breast epithelial cells to breast cancer cells, and the high expression of TRIM25 also suggests poor prognosis in breast cancer patients.