Background: Improving the position of the tooth is a major goal of orthodontic treatment. Removable orthodontics is an option in orthodontic treatment, because it is simple and removable and self-assembled. Removable orthodontics in addition to these advantages have a deficiency that can cause traumatiic ulcers in the soft tissues of the mouth. Objective of the Research: to investigate the incidence of traumatic ulcers in removable orthodontic appliance users. Research method: used descriptive observational with prospective cohort approach .The study was conducted at Dental and Oral Hospital of Universitas Muhammadiyah Yogyakarta (RSGM-UMY). The subjects of the study were users of removable orthodontic devices meeting the inclusion criteria. The number of research subjects as many as 32 people. Research results: Incidence of taumatic ulcers during the study were 18 cases. The incidence of traumatic ulcers according to age is most prevalent at the age of 20-25 years as many as 18 people (94.7%). The incidence of traumatic ulcers according to gender is the most common among women as many as 11 (57.8%). The incidence according to the time research is most prevalent in the first month as many as 9 people (28.1%). Conclusion: The incidence of traumatic ulcers in removable orthodontic device users at RSGM-UMY was highest in the first month after insertion of the device reached 28.1%. Most of the cases were women as many as 11 cases from 19 people and the age range of 20-25 years was 97.4%.