Purpose: To determine an appropriate Risk of Malignancy Index (RMI) cutoff value by comparative analysisof the four malignancy risk indices for distinguishing between borderline ovarian tumor (BOT) and ovarian cancer (OC). Materials and Methods: Retrospective analysis of the medical records of 339 patients (115 BOTsand 224 OCs). Results: There were no significant differences in the area under the ROC curve (AUC) for RMI 1, RMI 2, RMI 3, and RMI 4 (0.792, 0.791, 0.785, and 0.785, respectively). However, the diagnostic capability of the RMI was significantly greater than that of other factors. Conclusion: This study is the first to investigate the performance of the four Risk of Malignancy Indices for distinguishing between BOT and invasive OC. Although there were no significant differences between RMI scores, the RMIs were very effective at predicting an accurate preoperative diagnosis in patients with all BOT and OC histotypes.