Primary ovarian pregnancy is rare in the classifications of ectopic pregnancy with an incidence of 0.5-3% of all ectopic gestations. The surgical management of ovarian pregnancy is different from the operation of salpingocyesis. Preoperative diagnosis in early pregnancy can help to guide the optimal management. We have reported the case of a 37- year-old woman who presented with a naturally conceived ovarian pregnancy. Transvaginal ultrasound was used in this patient, and made the correct diagnosis of ovarian pregnancy before surgery. We found that transvaginal ultrasound can be helpful in the diagnosis of the location of ectopic pregnancies.