Abstract： Objective To investigate the effect of oxygenation index (PaO2/FiO2) on patients' prognosis through comparative analyzing the monitoring indicators of acute respiratory failure patients treated by invasive mechanical ventilation.Methods Data from 91 acute respiratory failure patients treated by invasive mechanical ventilation in respiration intensive care unit (ICU) of the General Hospital of PLA of Lanzhou from November 2006 to August 2011 were retrospectively analyzed.Patients were divided into survival group (n =55 ) and death group (n =36)by the outcome,the critical severity scores of the diseases and changes in blood gas analysis during ventilation were compared,and their correlation with prognosis were analyzed.Results There were no significant differences in acute physiology and chronic health evaluation Ⅲ (APACHE Ⅲ ) score,multiple organ dysfunction syndrome (MODS) score,acute lung injury (ALI) score between survival and death group before ventilation (62.77 ± 22.92 vs.74.62 ± 25.77,6.46 ± 2.45 vs.6.62 ± 3.03,1.90 ± 0.57 vs.2.10 ± 0.73,all P＞0.05 ).There was no significant difference in PaO2/FiO2(mm Hg,1 mm Hg=0.133 kPa) between survival and death group on the first day of mechanical ventilation ( 132.18 ±67.29 vs.139.24 ± 78.36,P＞0.05).PaO2/FiO2 in survival group were significandy higher than that in death group on the 3 days and 7 days of mechanical ventilation (3 days:205.47 ± 74.71 vs.149.76 ± 70.38,7 days:225.37 ± 67.20vs.120.94 ± 85.58,P＜0.05 and P＜0.01 ).Conclusions The present study demonstrated that the level of PaO2/FiO2 is related with the prognosis of acute respiratory failure patients treated by invasive mechanical ventilation.Continuously monitoring the changes in PaO2/FiO2 can be used as an important reference index to evaluate the prognosis of critical patients.