Primary retroperitoneal mucinous tumor is very rare. In the worldwide literature, only 14 cases have been reported. Herein, we report two cases of retroperitoneal mucinous tumor found at laparotomy. The patients were females, aged 24 and 21 years, respectively. They had bilateral normal ovaries. Histologically, one was mucinous cystadenoma and the other showed borderline histology. The connective tissue wall resembling ovarian stroma was noted in one case. The pathogenesis was discussed.