It is the intent of this study to simulate the performance of trans-esophageal aortic blood oxygen saturation (SeO2) monitoring by using trans-esophageal echocardiography (TEE) probe to provide an evidence for proper positioning of placing SeO2 sensor. 25 selected cardiac surgical patients were involved. After anesthesia was induced and trachea was intubated, the multiplane TEE probe was inserted into the esophagus, and the depths from the fore-tooth to the location for detecting aorta were recorded when the distances between TEE probe and aorta anterior wall were shorter than 0.5 cm, 1 cm, and longer than 1 cm, respectively. The o'clock directions of TEE probe were also recorded. The multiplane TEE probe, with its one-use pediatric SpO2 sensor attached, was inserted, and its anterior structure was examined when trans-esophagus oximetry detected high quality SpO2 signals. The results showed the beginning position of detecting aortic SpO2 signal in esophagus was mid esophagus (24.8 +/- 4.4 cm from the fore-teeth), the moving range was 5-10 cm downward, the deepest depth of detecting descending aorta was 41.4 +/- 4.7 cm from the fore-tooth, and the direction was left-posterior. And photoplethymography (PPG) wave's form was changed when SpO2 probe's anterior structure was different.