According to Health Promotion Administration, Ministry of Health and Welfare of Taiwan, the prevalence of periodontitis in Taiwan is nearly 90%. Periodontitis is one of the most neglected disease by people. Periodontitis often cause gum bleeding, clinical attachment level(CAL) loss and defect in alveolar bone, even worse tooth loosen. Dentists could repair these intra-bony defects by guided tissue regeneration(GTR) in clinical practice. This time, we present a case of localized periodontitis. We will discuss the timing and surgical method of GTR in repair of intra-bony defect and the effect to periodontal health of mandibular second molar by mandibular third molar impaction via the case report.