Background: Chronic obstructive pulmonary disease (COPD) is a major cause of chronic morbidity and mortality throughout the world. Exacerbation of COPD has negative effect on quality of life. Therapeutic effect of nebulized antibiotics in pulmonary infections has been reported previously. Hence, we evaluated the effect of nebulized gentamicin in acute exacerbation of COPD (AECOPD). Materials and Methods: In this clinical trial study, 86 hospitalized patients with AECOPD were divided into two groups for using nebulized gentamicin twice daily (case group) and placebo (control group) for 5 days in addition to standard treatment. On admission and on the 6 ^th day, respiratory rate (RR), white blood cell (WBC), spirometry, and  SPO² (arterial O² saturation by pulse oxymetry) were measured in groups. The severity of dyspnea was evaluated by the Medical Research Council scale. Results: In both groups, changes of SpO ₂ , RR, forced an expiratory volume of first second (FEV1), and forced vital capacity (FVC) were significant during the times of intervention (P < 0.05). However, changes of FEV1 and FVC were significantly different between two groups (P < 0.05). So that increments of FEV1 and FVC were higher in the case group than control group. WBC decreased significantly in the case group (P < 0.05) compared to control group. There was no significant difference between groups in severity of dyspnea after intervention (P > 0.05). Conclusion: Treatment with Nebulized Gentamicin in AECOPD exacerbation resulted in further improvement of FVC and FEV1 on the 6 ^th day.