In order to identify the effect of noninvasive positive pressure ventilation (NPPV) on decreasing inflammatory response, improving the pathophysiological manifestation and reducing the morbidity and mortality in the moderate acute respiratory distress syndrome (ARDS) patients, the investigators conduct this clinical trial comparing NPPV with invasive mechanical ventilation in more than twenty ICUs in China.

Inclusion Criteria:

1. acute onset
2. a clinical presentation of respiratory distress
3. arterial oxygen tension/inspired oxygen fraction (PaO2/FiO2) between 200 mmHg to 300 mmHg. The arterial oxygen tension will be measured in fifteen minutes after NPPV (CPAP 5 cmH2O) was used to the patients stably
4. presence of bilateral pulmonary infiltrate on chest radiograph
5. no evidence of left heart failure as assessed by echocardiography and/or a pulmonary artery wedge pressure( PAWP) < 18 mmHg

Exclusion Criteria:

1. age > 70 years or < 18 years
2. PaCO2 > 50mmHg
3. Glasgow Coma Scale (GCS)< 11
4. Upper airway/facial deformity or injury
5. pneumothorax or pneumomediastinum
6. unable to spontaneously clear secretions from their airway
7. respiratory arrest
8. severe ventricular arrhythmia or active myocardial ischemia
9. severe organ dysfunction (including gastrointestinal hemorrhage, disseminated intravascular coagulation, hepatic and renal dysfunction, SOFA score > 1)
10. severe chronic lung diseases(COPD, Asthma or ILD, et al)
11. end-stage patients who are expected to survive less than six months
12. severe abdominal distension
13. refuse to receive NPPV
14. unable to cooperate with NPPV