Background: COPD is known as the main cause of morbidity and mortality in the world. Morbidity in COPD patients is mainly due to the respiratory muscle dysfunction especially diaphragm and chest wall muscles. Respiratory muscle dysfunction is mostly seen in severe and progressive stages of the disease. COPD results in increased functional residual capacity (FRC). In severe cases of COPD, respiratory muscle dysfunction and FRC raise lead to the deterioration of hyperinflation. Materials and Methods: In our study, 30 COPD patients (28 males, 2 females) referred to Masih Daneshvari Hospital were evaluated. All cases were studied with the exact same body plethysmography equipment and the same technician. Results: The average age, height, weight and BMI of cases were 53 +/- 11 yrs,168.86+/-6.33 cm, 65.44+/-16.78 kg and 23.56+/- 6.32, respectively. The mean FEV1 according to the GOLD criteria was in the range of moderate to severe. Hyperinflation noted characterized by RV and reverse RV/TLC, was clearly noticed in our study (RV=225.9+/-82.11, RV/TLC%= 195+/-34.49). Conclusion: Based on our study results, there was a significant correlation between FEV1, hyperinflation (RV/TLC, RV), respiratory muscle function (PImax/P1) and respiratory time cycle Ti/Ttot .It should be mentioned that there was a significant correlation between PImax and Tension Time Index as well. (Tanaffos 2009; 8(3): 37-42)