Abstract： Objective To determine the optimal positive end-expiratory pressure (PEEP) according to inflation and deflation pressure-volume curve (P-V curve) in patients with acute respiratory distress syndrome (ARDS).Methods ARDS models were reproduced in 20 dogs,and they were randomly divided into two groups.In both groups,Levenberg-Marquardt iterative algorithm was employed using software to explore parameters fitting with Boltzmann formula,by which the real inflection point of pressure (Pinf d) in deflation limb or lower inflection point pressure (Plip) in inflation limb on P-V curve were defined.For the control group (inflation curve) P-V curve of Plip + 2 cm H2O ( 1 cm H2O =0.098 kPa) was applied as the best PEEP value.In the experimental group (deflation curve)the Pinf d was taken as the best PEEP value.The heart rate (HR),blood pressure (BP),fingertip pulse oxygen saturation (SpO2),static lung compliance (Cst),arterial partial pressure of oxygen (PaO2) and arterial partial pressure of carbon dioxide (PaCO2) were monitored at 0,2,6,12,24 and 48 hours.Results Oxygenation index increased significantly both in control and experimental groups.In experimental group,oxygenation index (mm Hg,1 mm Hg =0.133 kPa) of 12,24 and 48 hours was respectively significantly higher than that of the control group (12 hours:177.63 ± 8.94 vs.165.60 ± 8.90,24 hours:194.19 ± 10.67 vs.168.70 ± 10.60,48 hours:203.15 ± 13.21 vs.171.26 ± 9.21,all P＜0.01 ).Cst (ml/cm H2O) at 2,6,12,24 and 48 hours was respectively higher than that of the control group (2 hours:41.00 ± 4.17 vs.36.20 ± 3.90,6 hours:44.00 ± 4.65 vs.36.88 ± 3.39,12 hours:46.92 ± 5.47vs.37.92 ± 3.10,24 hours:42.83 ± 8.97 vs.37.92 ± 3.09,48 hours:42.64 ± 9.04 vs.37.97 ± 2.98,P＜0.05 or P＜0.01 ).Conclusion Determining optimal PEEP for ARDS with deflation P-V curve was better than that of inflation curve.