Objective To study the clinical features of primary signet ring cell carcinoma of prostate. Methods 2 cases of primary signet ring cell carcinoma of the prostate were studied and reviewed. Results The age of the 2 patients was 64 and 73.The clinical symptoms were dysuria,vesical irritability and perineum discomfort. Histologically, signet ring cell carcinoma was composed of round cells with abundant-clear cytoplasm and crescent-shaped nuclei on one side.Mitosis were frequently observed.Immunohistochemical testing showed the cancer cell was positive for prostate specific antigen (PSA),prostate acid phosphatase (PAP),AR,cytokeratin and negative for carcinoembryonic antigen (CEA),alcian blue/periodic acid-schiff (AB/PAS).One case (stage D)died 6 months after bilateral orchiectomy and flutamide therapy because of wide-spead metastasis;the other (stage B 2) has been surviving 25 months after radical prostatectomy,bilateral orchiectomy,endocrine therapy and local irradiation ministration. Conclusions Primary signet ring cell carcinoma of the prostate is extremely rare with a high grade of malignancy. The diagnosis depends on histopathologic and immunohistochemical studies.