Dyspnea defined as an uncomfortable sensation of breathing is the main cause of disability in chronic obstructive pulmonary disease (COPD) patients. There is evidence that the underlying mechanisms of dyspnea are multifactorial. The aim of this study was to investigate these mechanisms causing dyspnea in COPD patients and the relationship between functional parameters, dyspnea scales and quality of life questionnaire. For this purpose 56 patients (11 female, 45 male) were recruited. Pulmonary function tests including airflow rates, lung volumes, maximal respiratory muscle forces, diffusing capacity, breathing pattern, arterial blood gas analyses as well as dyspnea scales MRC, baseline dyspnea index (BDI) and The Saint George Respiratory Questionnaire (SGRQ) were performed. The overall group showed moderate obstructive disease (FEV1%= 59.02 +/- 3.30) and mild hypoxemia with some air trapping (RV/TLC%= 52.00 +/- 2.00). MRC scale did not show any significant correlation with pulmonary function parameters. There was significant positive correlation between BDI and airflow rates, PImax, DLCO and air trapping. Breathing pattern parameters (Ti/Ttot, VT/Ti) also correlated with BDI. There was positive correlation between PaO2 and BDI (p< 0.001). SGRQ scores correlated significantly with FEV1, PImax, RV/TLC and P 0.1. There was also strong correlation between BDI and SGRQ scores. In conclusion, dyspnea is the result of multiple factors such as airflow limitation, decreased respiratory muscle strength, changes breathing pattern, hypoxemia, and air trapping which in turn affects quality of life in patients with COPD.