Background: Ovarian immature teratoma is a very rare type of tumor associated with a high relapse rate and mortality. Correspondingly, early diagnosis is important for effective treatment. The goal of this study was to retrospectively analyze the ultrasound characteristics of 11 cases of immature teratomas. Materials and Methods: Between January 2002 and December 2010, 11 patients were diagnosed with pathologically confirmed immature teratomas. Patients enrolled in this study underwent a transabdominal ultrasound examination prior to surgery. Tumor size, shape, internal echo, calcification, cystic degeneration, and blood flow for each immature teratoma were analyzed. Levels of alpha fetal protein (&alphaFP) were also evaluated. Results: The average tumor size was 8 cm (range, 4–13). Echo patterns obtained included mixed echo (n = 7), solid echo (n = 2), and cystic echo (n = 2). Coarse calcifications were detected in five cases, while blood flow signal was detected in one case, and ascites were detected in three cases. Only one case had elevated levels of &alphaFP. Conclusion: Ultrasound imaging of immature teratomas detected large tumors that were predominantly involved mixed-echo masses, cystic degeneration, and coarse calcification on the cavity wall. In some cases, blood flow signal was detected in the mass and ascites were present in the abdominal cavity.