Background: There is no data on how long time of sweep gas off is enough for assessing the adequacy of lung function to make a decision of decannulation in patients receiving venovenous extracorporeal membrane oxygenation (ECMO). The objective of this study was to investigate the arterial blood gas changes during the sweep gas off trial and the clinical outcomes depends on the duration of the trial off. Methods: A retrospective observational study using prospectively registered data of 74 consecutive patients receiving venovenous ECMO who had weaning trial at least once Results: During the trial off, significant change of blood gases were observed within 1 hour in all patients. PaO2 and PaO2/FiO2 ratio were significantly lower in failed trials than successful trials within one hour after stopping sweep gas. After 1 hour of the trial off, however, there was no significant change in blood gases until the end of the trial. Finally, 60 patients successfully weaned from ECMO and divided into two groups according to the duration from the trial off to decannulation: for 2 hours or less and more than 2 hours. However, there was no significant change in ventilator settings and blood gases immediately and 24 hours after decannulation in the two groups. In addition, there was no patient who needed ECMO support again or died within 24 hours after decannulation in the two groups. Conclusions: Blood gases were not changed from 1 hour after stopping the sweep gas and the clinical outcomes depends on 2 hours of trial off was not different in patients receiving venovenous ECMO.