Objective To study the effects of hemodilution on oxygenation during one-lung ventilation(OLV).Methods Forty patients undergoing lung surgery with or without chronic obstructive pulmonary disease（COPD）were enrolled.The study was performed in the supine position before surgery.The tracheas were intubated with a double-lumen tube.OLV was initiated for 15 min.After 15 min of OLV，arterial and venous blood gas samples were collected and analyzed.The cardiac output (CO) was measured.Two-lung ventilation was reinstituted，and hemodilution was performed (6% hydroxyethyl starch，10 mL/kg).Subsequently，OLV was performed again for 15 min.Then arterial and venous blood gas samples were collected and analyzed.The cardiac output (CO) was measured.Results Hemodilution resulted in a significant and similar decrease in HB concentration in patients both with or without COPD.However，hemodilution resulted in a significant decrease in PaO2 in COPD patients rather than subjects without COPD.Conclusion Mild hemodilution impairs gas exchange during OLV in COPD patients.