Objective To assess the clinical efficacy of neoadjuvant chemotherapy （NAC） for breast invasive ductal carcinoma with MR diffusion weighted imaging. Methods Thirty patients with breast invasive ductal carcinoma underwent conventional MRI scanning and diffusion weighted imaging examination before and after preoperative neoadj-uvant chemotherapy. Two experienced radiologists independently analyzed and measured the maximum lesion diameter and apparent diffusion coefficient （ADC） values before and after treatment，respectively. Statistical analysis was performed for testing the tumor maximum diameter and ADC values change by using the paired t-test. Results After NAC treatment，the maximum tumor diameter of invasive ductal breast carcinoma sharply reduced〔（4.33±0.83） cm vs. （2.04±0.64） cm，P＜0.001〕. When b value was 1 000，the mean ADC values of breast massess were significantly changed after NAC treatment〔（1.89±0.15） ×10-3mm2/s vs. （1.14±0.31） ×10-3mm2/s， P＜0.05〕. Conclusion MR diffusion weighted imaging can non-invasively and accurately assess the NAC efficacy， which are helpful for making surgical strategies.