Objective To study the value of the maximum diameter of tumor in evaluating the therapeutic effect of neoadjuvant chemotherapy for locally advanced breast cancer (LABC). Methods 127 women with LABC underwent dynamic CEMRI examination before chemotherapy, after the 2nd cycle and the 4th cycle of chemotherapy. All patients were divided into major histological response group (MHR) and non-major histological response group (NMHR) according to the final pathologic response. The response to NAC before surgery evaluated by the variance of tumor size was compared to pathologic response after surgery. ROC curve analysis was performed to assess the effect of the maximum diameter and the variance of tumor size (⊿D %) in predicting the pathologic response to the NAC treatment. Results 127 cases of LABC patients were divided into a MHR group of 68 cases and a NMHR group of 59 cases. The response to NAC after 2nd and the 4th cycle of NAC before surgery evaluated by RECIST criterion was significant difference to pathologic response(P<0.05) after surgery. Az (area under curve) of The maximum diameter variance rate of the tumor after 2nd and 4th cycle were 0.66 and 0.73 respectively. The best diagnose cut off were variance rate of 42.68% and 58.90% respectively. Conclusion The maximum diameter variance rate of the tumor after 2nd cycles of NAC may be useful in predicting the final pathological response of breast cancer to NAC treatment, but which after the 4th cycle would be more efficiency. Take the variance rate greater than 58.90% as diagnostic criteria to definite the response level can improve the diagnose accuracy.