Objective To investigate the value of 3.0T MR diffusion-weighted imaging (DWI) in diagnosing the renal pelvic cancer and the association of apparent diffusion coefficient (ADC) value with the histological grade and clinical stage of cancer. Methods The clinical data of 33 patients with renal pelvic carcinoma were retrospectively analyzed. Routine MRI and DWI with b-values of 0 and 800 s/mm² were performed, and the examination results were compared with the corresponding pathological results. We then calculated the sensitivity, specificity, accuracy, positive and negative predictive values of DWI. The DWI signal characteristics of lesions were recorded and analyzed, and three regions of interest (ROIs), large and small ROIs and minimum ADC (minADC) value, were selected for each lesion. The differences of ADC values between different genders, locations, cancer tissue and normal renal parenchyma, among three ROIs, among different pathological grades, among different clinical stages were compared and analyzed. Immunohistochemical staining was used to detect the expression of Ki-67 in renal pelvic cancer tissues, and the relationship between ADC value and Ki-67 expression was also analyzed. Results The results of DWI with 800 s/mm² b-value showed that the sensitivity, specificity, accurate rate, positive and negative predictive value for renal pelvic carcinoma were 90.9%, 91.4%, 91.2%, 90.9% and 91.4%, respectively. The ADC values were significantly different between normal renal parenchyma and renal pelvic carcinoma tissues (P<0.05), and they were not significantly different between different genders, locations or among small ROI, big ROI and the minADC values. The ADC value of low grade (G₁-G₂) tumors was significantly higher than that of high grade (G₃) ones (P<0.01). The ADC value of cancer cells confined to the renal pelvis (T_a-T₂) was higher than that with local invasion (T₃)(P<0.05). Ki-67 expression was found associated with the pathological grades and tumor stages of pelvic carcinoma, and there was a negative correlation between ADC value and Ki-67 expression (r=-0.88, P<0.01). Conclusion Our findings suggest that the size of ROI may be of little value in diagnosis of urothelial carcinoma, and DWI may contribute to the preoperative assessment of histological grade and clinical stages of renal pelvic carcinoma.