Objective This study systematically investigated alterations of ADC value before and after the first cycle of neoadjuvant chemotherapy(NAC)in patients with HER-2 overexpression subtype of breast cancer,and alterations of ADC value were compared with postoperative pathology studies in order to establish the role of DWI on early predicting breast cancer chemotherapy efficacy.Methods 36 patients with locally advanced Her-2 overexpression breast cancer undergoing 1.5T MRI were recruited.All patients had complete pathological findings before and after surgery.Breast MRI were performed before and after first cycle of NAC.ADC values were measured before and after tumor NAC,respectively.Then the changes of ADC(△ADC)values were calculated.Pathological results after tumor resection was recognized as the golden standard.The sensitivity and specificity of ADC were determined by ROC curve,and cutoff value of ADC was presented.Results 21 cases of patients were effective under NAC,while other 15 cases were ineffective.The area under the △ADC curve was 0.968,had an excellent diagnostic effect.Conclusion The changes of ADC before and after the first cycle of NAC can early predict the whole cycle of NAC effectiveness,and had the high sensitivity and specificity.Furthermore,DWI may be considered as an imaging biomarker candidate for predicting the efficacy to neoadjuvant chemotherapy of this tumor.