OBJECTIVE: During total arch replacement (TAR) using frozen elephant trunk (FET) technique with Frozenix for true thoracic aortic aneurysm (tTAA), oversized FET tends to be chosen similar to the endovascular devise selection. However, the oversized FET is considered a risk factor for intimal injury. The appropriate size selection of FET remains insufficiently understood. METHODS: Between October 2014 and March 2022, a total of 49 patients underwent TAR using Frozenix for tTAA. Out of 49 patients, four patients planned to staged surgery were excluded, 19 patients were operated on with an undersized Frozenix compared with the descending aorta (undersized FET group) and in 26 patients an equal or oversized Frozenix was used (oversized FET group). Clinical outcomes and postoperative diameter changes were investigated. RESULTS: In-hospital mortality was 0%. The mean diameter of Frozenix and the descending aorta was 30.7 mm and 28.8 mm, respectively, in the oversized FET group, and 26.7 mm and 30.1 mm in the undersized FET group. Postoperative computed tomography (CT) demonstrated no endoleaks not only in the oversized FET group but also in the undersized FET group. CT also revealed that undersized FET had expanded more than the original diameter in all cases except for two, with an average of 2.47 ± 1.53 mm. Additionally, the descending aorta covered with Frozenix shrank in 10 patients (53%). Postoperative adverse aortic events were not observed. CONCLUSIONS: Undersized Frozenix tightly fit the descending aorta and resulted in complete sealing without endoleaks. Oversized FET is not strictly necessary considering the size-related adverse complications.