It is known that the pathophysiology of chronic obstructive pulmonary disease (COPD), a disease that develops against inhaled harmful chemicals and is characterized by progressive airway obstruction, is associated with oxidative stress. However, due to inconsistent findings, the oxidative status of COPD is not fully understood. It is thought that determining the oxidative state in detail may be an effective diagnostic criterion in the diagnosis of COPD. In addition, supplementing antioxidative mechanisms with diet or drugs is among the new treatment strategies. Total antioxidant status (TAS), total oxidant status (TOS), paraoxonase (PON1), arylesterase (ARES), total thiol (THIOL) levels were examined in COPD and control. Oxidative stress index (OSI) ratios were calculated. Oxidative balance did not change according to healthy individuals, although we observed a tendency to increase together in oxidative and antioxidative parameters in COPD patients. We observed that the tendency to increase antioxidative capacity in COPD patients is independent of PON1, ARES, and THIOL. When compared to the control group, the serum thiol parameter of the COPD group was shown to be significantly low. We think that the decrease in serum thiol parameter can be considered as a new indicator to confirm the diagnosis of COPD.