Objective: Multidrug resistance (MDR) to chemotherapeutic drugs is an important reason for clinical chemotherapy failure. So far, the relationship between FOS-like antigen1 (FOSL1) and chemotherapy sensitivity of breast cancer remains unclear. This study investigates the relationship between FOSL1 and chemotherapy sensitivity of breast cancer and its molecular mechanism. Methods: Doxorubicin-resistant MCF-7/ADR breast cancer cells were transfected with NC (control) or FOSL1 siRNA and assayed for cell viability and relative colony number by MTT assay and colony formation, respectively. The expression level of FOSL1 was detected by immunohistochemistry (IHC). The relationship between FOSL1 and chemotherapy sensitivity was analyzed by a one-way of variance analysis and Pearson’s chi-square test among a total of 50 patients with stage II and III breast cancer before and after they received epirubicin-based neoadjuvant chemotherapy (NCT) between 2012 and 2017. Results: The expression of FOSL1 was increased in breast cancer tissues compared with normal breast tissues (P<0.05), and the expression of FOSL1 was decreased after NCT treatment compared with breast cancer tissues (or before NCT). This lower expression of FOSL1 was correlated with chemotherapy resistance or chemotherapy sensitivity (P<0.05). Moreover, the expression level of FOSL1 was markedly lower in NCT-sensitive patients than that of NCT-resistant patients (P<0.05). Conclusion: Down-regulation of FOSL1 potentiated chemotherapy sensitivity of breast cancer, and its lower expression attenuated chemotherapeutic drug resistance in human breast cancer cells. FOSL1 might be a drug target for predicting chemotherapy effect in breast cancer.