Vitamin D has several extra calcemic effects. Vitamin D deficiency is highly prevalent in chronic obstructive pulmonary disease (COPD) patients but little is known about it’s association with lung function. Objective: To investigate whether supplementation with vitamin D could improve pulmonary function in COPD patients. Design: Before and after, double center, clinical trial. Setting: Hazrat Rasoul University Hospital, Tehran, and Imam Khomaini University Hospital, Ahvaz, Iran. Participants: 24 patients with mild to very severe COPD. Intervention: Loading dose of 300,000–600,000 International Units (IU) of vitamin D, then 50000 IU weekly for 12 weeks. Measurements: The outcomes included forced expiratory volume in one second (FEV1), forced vital capacity (FVC), vital capacity (VC), forced expiratory flow between 25%–75% of forced vital capacity (FEF 25%–75%), exercise capacity according to the six minute walk test(6MWT) and the saturation of oxygen during exercise. Results: The mean FEV1 (p
-value = 0.866), FVC (p
-value = 0.475) and VC (p
-value = 0.425) were not significantly different before and after intervention. FEF 25%–75% did not improve with this intervention (p
-value = 0.555). The vitamin D supplementation did not have any significant effect on the exercise capacity (p
-value=0.175) or the saturation of oxygen (p
-value = 0.635). Conclusion: Pulmonary function and exercise capacity did not improve with vitamin D supplementation in COPD patients.