Arterial hypoxemia in the postoperative period is of particular importance hecause it may delay recovery, exacerbate organ dysfunction, and contribute to motality. Oxygen supply during recovery period is to prevent the complications attributed to hypoxemia, including confusion, cardiac arrythmia, reactive pulmonary hypertension, lactic acidosis and tissue death. A low flow oxygen supply device, O2-Cap, is used to supply oxygen in postoperative patients. However, it was hypothesized that increase of inspired oxygen fraction (F1O2) in an O2-Cap is less than that in a T-Piece with an expiratory limb served as a reservoir. The purpose of this study was to compare the effects of O2-Cap and T-Piece on the increase of arterial tension (PaO2) during early postoperative period. In a prospective study, we measured vital signs, PaO2, and arterial carbon dioxide tension (PaCO2) in 24 adult patients (ASA class I or II) breathing room air in the operation room. After aMominal surgery, they were transferred to the recovery room with the endotracheal tube (intemal diameter, 7.5 mm) whose cuff is deflated. We devided the patients into two groups in the same number. Group C received O2 of 5 1/m via an O2-Cap and group T via a T-piece. Postoperative values were obtained at 5 min, 30 min, and 60 min in the recovery room and were compared between two groups. Preoperative values were served as controls in each group. Vital signs were similar in the two groups. PaCO2 in both groups maintained normal ranges. PaO2 in group C increased significantly to 132.5+/-28.1 mmHg, 134.7+/-26.8 mmHg, and 136.7+/-23.6 mmHg at 5 min, 30 min, and 60 min after operation, respectively, compared with preoperative value, 99.7+/-10.0 mmHg (p<0.05). Group T also showed a significant increases in PaO2 from 102.4+/-18.2 mmHg, preoperatively to 223.1+/-56.1 mmHg, 267.0+/-90.0 mmHg, and 249.8+/-76.5 mmHg at 5 min, 30 min, and 60 min after operation, respectively (p<0.05). PaO2 in group T increased more than that in group C during the same period after the operation. It is concluded that T-Piece with a reservoir increases PaO2 more than O2-Cap andmaintains normal range of PaCO2 without hemodynamic instability. Therefore it is expected that T-Piece with reservoir of 70 ml is safe and effective in spontaneously breathing intubated patients for oxygen supply during postoperative period.