We retrospectively assessed the dual-time-point (DTP) F-18 FDG PET/CT findings of thymic epithelial neoplasms (TENs) and investigated the diagnostic capacity of PET/CT compared to that of CT for predicting carcinoma. We calculated the ratio of the standardized uptake value of the tumor and that of the aortic arch (T/M ratio) for both the 90-min early scan and the 2-h delayed scan in 56 TEN patients. We used a multivariate logistic regression (MLR) analysis to estimate the CT features of carcinoma. We compared the diagnostic capacities of PET/CT and chest CT using receiver operating characteristic (ROC) analyses. The ROC curve revealed that the appropriate cut-off T/M ratio value for the highest accuracy was 2.39 with 75.0% accuracy. The area under the curve (AUC) was 0.855. The statistical analyses for DTP scans of 35 TEN patients demonstrated 74.3% accuracy and 0.838 AUC for the early scan versus 82.9% and 0.825 for the delayed scan. The MLR analysis indicated that mediastinal fat infiltration was a predictor of carcinoma. The ROC curve obtained for the model yielded an AUC of 0.853. Delayed scanning could improve the diagnostic capacity for carcinoma. The T/M ratio and mediastinal fat infiltration are predictive of carcinoma with moderate diagnostic accuracy.