Breast cancer is characterized with poor prognosis and high recurrence. HER2 is highly expressed in breast cancer and is a target for cancer therapy and prevention. Here, we investigated the anti-tumor activity of the combination of an HER2 inhibitor, trastuzumab with an EGFR-inhibitor, nimotuzumab in HER2-overexpressing breast cancer. Our data showed that a greater anti-tumor activity from the combination of trastuzumab and nimotuzumab than any alone usage of above antibody both in vitro and in vivo. Based on the combination index value, our data demonstrated that nimotuzumab synergistically enhanced trastuzumab-induced cell growth inhibition. Furthermore, we investigated the possible mechanism of this synergistic efficacy induced by trastuzumab plus nimotuzumab. Data showed that the combination was more potent in reducing the phosphorylation of HER2 and ERK1 /2. We also found that the synergistic inhibition was partly attributed to the ROS generation and repression of NRF2 pathway that is known to promote cell growth. These results support the clinical development of this two-drug regimen for the treatment of HER2-amplified breast cancer. (C) 2017 Elsevier Inc. All rights reserved.