Young permanent teeth could have pulp necrosis so that root growth will stop, meanwhile the tooth apex is still open, and the root canal is wider at the apex than the cervical region. The goal of Apexification treatment is to stimulate the process of forming the apex of the tooth by formation of hard tissues at the tooth apex. The purpose of this article was to report an apexification treatment case in a 12 years old child who had necrosis on tooth 22 with open apex. Apexification treatment was performed using calcium hydroxide paste. Based on the radiography examination on two months after the treatment, the tooth apex begin to calcified, radiopaque patches were appear at the tooth apex but there was radiolucent area that still remain. Five months after apexification, from the radiograph seemed that the tooth apex has been close (calcified) entirely, which was characterized by radiopaque on the tooth apex with blunderbuss shaped. Root canal treatment (pulpectomy) and tooth filling was done afterwards. Radiograph evaluation 12 months after the treatment showed good condition of tooth apex (radiopaque), there is no radiolusent area on periapical which was a pathological signs. On this case, the apexification treatment using calcium hydroxide paste demonstrated succeed in closing the tooth apex after 5 months. Evaluation on 12 months after the treatment showed good condition of tooth apex with no pathological signs.