A 28-year-old woman realized a left breast tumor. Mammography and ultrasonography revealed focal asymmetric density on the MI area(category 3)and a low-echoic 10 mm tumor with unclear boundaries. We performed an US-guided breast biopsy to confirm the diagnosis. The histopathological examination result suspected nodular fasciitis; however, borderline phyllodes tumor cannot be denied. Tumorectomy was performed under general anesthesia. The final histopathological examination revealed nodular fasciitis due to amplifying fibroblasts with irregular directions. Nodular fasciitis is a benign lesion and sometimes disappears spontaneously. Tumorectomy is often needed to confirm the diagnosis that cannot be identified by needle biopsy. We report a case of breast nodular fasciitis needed to differentiate from borderline phyllodes tumor.