A 7-year-old girl with asthma developed large mucus plug confirmed by bronchoscopic evaluation. Her respiratory failure and concomitant multiple atelectasis were improved after bronchoscopic removal of the mucus plug. Mucus plug is known to frequently occur in children with asthmatic attack, because of exudate accumulation in airway, different profile of mucus in asthmatic conditions and decreased activity of cilia. Despite the conventional concept that mucus plus is managed by chest physiotherapy, large mucus plug in asthma may cause severe respiratory failure with the development of atelectasis which should be removed by bronchoscopy. We report herein a case of large mucus plug that caused atelectasis in a child with asthmatic attack.