A case of a endodermal sinus tumor of the ovary in a patient with 45, XO/46, X, dic(Y) mosaicism is reported because of the rarity of the karyotype and condition. A 15-year-old girl was admitted to Yamagata University Hospital because of constipation for several days. Physical examination showed webbing of the neck, cubitus valgus and short stature. Her abdomen was bulging. Chromosomal analysis showed 45, XO/46, X, dic(Yq) mosaicism in karyotype. α-Fetoprotein and CA-125 in the serum were high. A left ovarian tumor was found by laparotomy; however, the right ovary was a streak gonad and the uterus was hypoplastic. An endodermal sinus tumor was diagnosed by a pathologist. After operation, cisplatin-vinblastin-bleomycin chemotherapy was instituted and the tumor marker went down. This patient is still healthy and under observation at the outpatient clinic.