Objective To investigate the physiological effects of different oxygen injection site on ventilatory status and oxygenation during noninvasive positive pressure ventilation ( NPPV) with portable noninvasive ventilators. Methods A prospective crossover randomized study was performed. Oxygen injection site was randomized into the outlet of the ventilator, the connection site between mask and circuit, and the mask under the condition of leak port immobilized in the mask. Oxygen flow was retained in the baseline level at the initial 5 to 10 minutes, and adjusted to obtain arterial oxygen saturation measured by pulse oximetry ( SpO2 ) ranging from 90% to 95% after SpO2 remains stable. SpO2 at the initial 5 to 10 minutes, oxygen flow, ventilatory status, oxygenation, hemodynamics and dyspnea indexes at0. 5 hour, 1 hour, and 2 hours of NPPV were compared between different oxygen injection sites. Results 10 patients were recruited into the study. Under the condition of the same oxygen flow, SpO2 with oxygen injection site in the outlet of the ventilator was significantly higher than that with oxygen injection site in the connection site between mask and circuit [ ( 98.9 ±0.9) % vs. ( 96.9 ±1.1) % , P =0. 003] , whereas SpO2 with oxygen injection site in the connection site between mask and circuit was significantly higher than that with oxygen injection site in the mask [ ( 96.9 ±1.1) % vs. ( 94.1 ±1.6) %, P = 0.000] . Oxygen flow with oxygen injection site in the mask was statistically higher than that with oxygen injection site at other sites ( P lt; 0.05) . Arterial oxygen tension/ oxygen flow with oxygen injection site in the outlet of the ventilator was significantly higher than that with oxygen injection site in the connection site between mask and circuit ( 67.9 ±31.1 vs. 37.0 ±15.0, P =0.007) , and than that with oxygen injection site in the mask ( 67.9 ± 31.1 vs. 25.0 ±9.1, P = 0.000) . pH, arterial carbon dioxide tension, hemodynamics and dyspnea indexes were not significantly different between different oxygen injection sites ( P gt; 0.05) .Conclusions When portable noninvasive ventilator was applied during NPPV, oxygen injection site significantly affects improvement of oxygenation, and shows a trend for affecting ventilatory status and work of breathing. When the leak port was immobilized in the mask, the nearer oxygen injection site approaches the outlet of the ventilator, the more easily oxygenation is improved and the lower oxygen flow is demanded.