Background: chronic obstructive pulmonary disease (COPD) is currently the fourth leading cause of death in the world, and it is projected to be the third leading cause of death by 2020. More than 3 million people died because of COPD in 2012, accounting for 6% of all deaths globally. Objective: the aim of this study is to assess the diaphragmatic function in COPD patients using the ultrasonographic technique, and to study its correlation with severity of the disease. Patients and Methods: this study was carried out during the period from November 2018 to June 2019, on sixty patients with clinically stable chronic obstructive pulmonary disease (COPD), during their follow up in the outpatient clinic of Chest Department, Bab-Al-Sha'reia University Hospital. Results: thickness of the diaphragm (TD) at different lung volumes and capacities (RV, FRC and TLC) estimated by U/S, was found to be progressively decreased with increasing COPD severity. TD was found to be decreased significantly in COPD patients when compared with controls. The only exception was the presence of a non-significant relationship between TDRV in control and mild COPD groups, which may denote that diaphragmatic thickness is not markedly affected in early COPD at low lung volumes. Conclusion: U/S is a simple, easily learned, non-invasive and reliable method that can be used in assessment of the diaphragmatic function and kinetics. There is a significant negative correlation between diaphragmatic function (assessed by measuring diaphragmatic thickness and excursion through U/S) and COPD severity.