Background and Aim: Evaluation of the treatment OUTCOME is an important stage in every healthcare system, including orthodontics. This study aimed to assess the attitude and practice regarding treatment OUTCOME ASSESSMENT (TOA) among Iranian orthodontists. Materials and Methods: In this cross-sectional survey, a questionnaire was developed, which consisted of demographic, attitude, and practice sections. Content validity was evaluated using content validity index (CVI), and reliability of the answers was evaluated using test-retest method and Kappa statistics. The online questionnaire was sent to a Telegram group including 315 Iranian orthodontists and orthodontic residents. Only graduated orthodontists with more than two years of experience were included. Results: The average CVI was 0. 95, and Kappa value was greater than 0. 6. Totally, 66 orthodontists participated in the survey. Almost all orthodontists acknowledged the importance and positive effects of TOA. Seventy seven percent and 87. 9% of the participants stated that treatment duration and stability, respectively, should be considered while evaluating treatment OUTCOME. The most important disadvantage of TOA was its time-consuming nature (56. 1%). The best criterion for the evaluation of treatment OUTCOME was “ treatment objectives” (89. 4%). Photographs (84. 8%) were the most common post-treatment document followed by panoramic radiographs (72. 7%). Also, 23% and 65. 2% of the participants “ always” or “ usually” performed TOA, respectively. TOA was mostly performed subjectively (84. 8%) and during the debonding session (48. 5%). Conclusion: The results indicated that almost all orthodontists participating in this survey were aware of the importance of ASSESSMENT of treatment OUTCOME. Most of them usually performed such an ASSESSMENT using subjective methods.