Self-injurious behavior is a serious problem that is commonly seen in developmentally disabled individuals. Lip-biting is considered a common type of self-injurious behavior. There have been no standard methods established to manage the behavior. A variety of oral appliances were presented to prevent the oral trauma. We reported a case of lip-biting on a 2-year-old girl with cerebral palsy and mental retardation. Through the use of mouthguard, it got a quick and effective treatment to inhibit this behavior. We conclude that mouthguard is a rapid and effective treatment modality in self-injurious lip-biting.