Abstract： Objective　To investigate the effects of extrinsic positive end-expiratory pressure (PEEPe) on work of breathing in patients with chronic obstructive pulmonary disease (COPD) and their corresponding mechanism.Methods　Ten ventilated patients with COPD were included in the study. A Bicore CP-100 pulmonary monitor （Bicore Monitoring System, USA） was used for monitoring respiratory mechanics. First, dynamic intrinsic positive end-expiratory pressure (PEEPi,dyn) was measured when PEEPe was zero, which was called PEEPi,dynz. Then the PEEPe was set randomly at 0%, 40%, 60%, 80% and 100% of PEEPi,dynz respectively. Pulmonary mechanics and other parameters (heart rate, blood pressure and blood gas analysis) were measured 30 minutes after the level of PEEPe was changed.Results　Work of breathing patient (WOBp), pressure time product, difference of esophageal pressure and PEEPi,dyn decreased significantly when PEEPe was applied, and continued decreasing as PEEPe was increased. Work of breathing ventilator increased significantly when PEEPe was increased to 80% and 100% of PEEPi,dynz. Significantly positive linear correlation was found between the changes in WOBp and in PEEPi,dyn.Conclusions　WOBp decreases gradually as PEEPe is increased. WOBp decreases by narrowing the difference between the alveolus pressure and the central airway pressure at the end of expiration when PEEPe is applied.