Ewing's sarcoma/primitive neuroectodermal tumor (EWS/PNET) in the kidney is a rare but high-grade malignant tumor that affects predominantly elder children and adolescents. Patients mostly present with nonspecific symptoms such as abdominal pain and gross hematuria. Since EWS/PNET has a rapid clinical progression with early metastasis and death, it is essential to make an accurate and early diagnosis. Once diagnosed, multimodality treatment, including radical surgery combined with adjuvant chemotherapy, and radiotherapy if necessary, is recommended. Unfortunately, there are no characteristic signsthat have been described in ultrasonography or any other imaging modalities so far. The diagnosis of EWS/PNET is now based on a classical histological and immunohistochemical investigation complemented by a demonstration of specific chromosomal changes. Strong immunoreactivity to CD99 is ubiquitous, and t(11;22) translocation is seen in approximately 90% of EWS/PNET. Herein, we report a patient with such condition. The patient was a young woman, and she presented with sudden right flank pain clinically. Ultrasonography revealed a large heterogeneous mass in the lower pole of her right kidney. The tumor compressed the renal pelvis and led to upper pole caliectasis. Color Doppler demonstrated blood flow with a pulsatile arterialized waveform within the mass. The patient received radical nephrectomy with right renal vein and vena cava thrombectomy. A search for other sites of tumor involvement yielded negative results. And six cycles of chemotherapy were sequentially performed. The diagnosis of EWS/PNET was confirmed based on primitive small round cell histology and characteristic immunohistochemical results. She was still alive with no evidence of recurrence five years after initial diagnosis. We would like to point out that ultrasound is still a useful method for initial assessment, and ultrasound-guided fine needle aspiration may play an important role in determining preoperative diagnosis.