Introduction: Temporomandibular joint ankylosis is fusion of the head of mandibular condyle to the glenoid fossa of temporal bone at skull base. Untreated ankylosis causes distressing conditions such as speech impairment, difficulty in chewing, poor oral hygiene, malocclusion, facial disfigurement, and compromise of the airway. The two main causes are trauma and infection in or around the joint regions. Methodology: A retrospective cross sectional study was conducted in 25 patients (n=14 male and n=11 female) with mean age of 21.9(ranged 6-50) diagnosed with Temporomandibular joint ankylosis at Addis Ababa University, Oral and Maxillofacial Surgery affiliate Hospitals both Yekatit 12 Hospital medical college and St. Peter specialized Hospital. Data was collected from patients’ medical records registered in a period of 3 years from January 2017 to December 2019. Epidemiological information (EPI- INFO 7) computer software was used for data analysis. Result: The study results revealed trauma was the most common cause of TMJ ankylosis representing (80%) of all causes. Males were more affected (56%) than females. The highest incidence of ankylosis was between the age of 11 and 20 (40%). Unilateral ankylosis was reported in (60%) and (68%) was bony ankylosis based on tissue involved. A total of (40%) of the patients were treated by interposition gap arthroplasty by using temporal muscle and fascia as an interposition material. Conclusion: The findings of this study conclude trauma was the commonest cause of ankylosis. A proper screening of childhood injuries and early referral to maxillofacial centers recommended ensuring proper evaluation of specific post traumatic aetiologic factors and for early treatment to prevent ankylosis.