Self-injurious behavior (SIB) has been defined as the deliberate destruction or alteration of body tissue without conscious suicidal intent. It occurs in conjunction with a variety of psychiatric disorders as well as various developmental disabilities and some syndromes. The behavior is destructive and causes concern and distress to all involved in the care and treatment of the affected individual. A 13-year-old girl with autism, mental retardation and delayed development was reffered from her pediatrician because of severe and painful lower lip biting. An intraoral examination revealed a diffuse swelling of lower lip. It was covered with necrotic slough and the ulcer and scarring of the lower lip was observed. We chose to use an oral removable prosthesis for Conservative treatment. It was decided to use a soft silicone mouthguard in the maxillary arch. Initially, she could not tolerate the appliance inside her mouth but soon adapted with the appliance. After one month, she lost the mouth guard and started lip biting. So we made mouth guard again. There are no standard methods for preventing self-injurious behavior in a patient who is developmentally disabled. Appropriate preventive methods must be developed for each individual patient based on close observation and clinical findings. Behavior modification techniques, pharmacological treatment, extraction of teeth, orthognathic surgery and intra/extra oral appliances can be performed for adjust self-injurious behavior. A suitable oral guard could be tried initially before employing more invasive approaches.