Perpose: Inflammtory breast cancer(IBC) is the most lethal subgroup of primary breast cancer with poor prognosis. And there remain difficulties to distinguith it from acute mastitis either in the clinical manifestation, or imaging festures. Patient and Methods: A 40-year-old woman patient was followed up with enlargement, tenderness in her left breast after traumatism on it. Physical examination revealed that the whole left breast was swelling, tenderness, dermal redness, temperature increasing. MR mammography was performed by using 3.0-T MR system(Signa HDx, GE healthcare) with a dedicated 8-channel breast array target coil, including pre-contrast T1,T2-weighted imaging and multiphase contrast-enhancement. MR imaging data was processed in AW4.4 workstation. Results: MRM revealed that the left breast obviously enlarged with diffuse skin thickening and axillary lymphadenectasis. The lesions showed hyperintense compared to the contralateral breast in T2-weighte images. Signal intensity increased significantly in the early postcontrast phase, and time-intensity curve was wash-out and plateau sign. Core-needle biopsy was performed after antibiotic treatment for 5 days, and histopathology proved that it was infiltrating ductal carcinoma with tumor emboli within the lymphatic vessels. Conclusion: IBC is difficult to diagnose and often mistaken for acute mastitis. Dynamic contrast-enhanced MRM is useful in differential diagnosis.