This case demonstrates external root resorption arrest and formation of an apical barrier by conventional endodontic therapy combined with calcium hydroxide dressing and a mineral trioxide aggregate (MTA) apical plug. The upper left central incisor exhibited severe external root resorption, and a periapical radiolucent lesion received root canal treatment combined with calcium hydroxide dressing for over 4 years. The dressing was renewed every 3 months, and the coronal portion was sealed with composite resin. After detection of the apical barrier, a MTA apical plug backfilled with gutta-percha was performed and the access carity was restored with composite resin. The patient has been regularly recalled every 6 months, and no symptoms or signs hare been noted. Radiography showed the apical lesion healed and the external root resorption had been arrested after 1 year.