Background: Autism Spectrum Disorder is a neuro developmental disorder which has a trend in its increasing prevalence globally. Measuring the outcome of intervention will have a great value for parents and clinicians. At present, there are no tools validated in the Sri Lankan setting to assess treatment outcomes in Autism Spectrum Disorder. Autism Treatment Evaluation Checklist (ATEC) is a self-administered questionnaire in Sinhala for parents/caregivers that is a feasible method of evaluating outcome in a busy clinic.Aims: This study was conducted to translate and culturally adapt the ATEC to the Sinhala language and to establish the validity and reliability of the translation to the local setting.Methods: Permission was sought from the developers of the ATEC questionnaire and ethical approval was obtained for the study. Study was conducted in two stages. In the first stage, ATEC was translated using the translation-back translation method. An expert panel produced a pre-final questionnaire which was pre-tested prior to formulating the final Sinhala version of ATEC. The second stage of the study was conducted to establish validity and reliability of Sinhala ATEC questionnaire in the Sri Lankan popu-lation. Content validity was assured in each step of the translation by the expert committee. Construct validity of the Sinhala ATEC questionnaire was assessed by comparing the sample of 119 parent/caregiver filled final version of the Sinhala ATEC score against the Consultant Child and Adolescent Psychiatrist assessment of the same children conducted independently and recorded according to DSM-5 (level 1, 2 or 3). Test-retest reliability was determined by the same parent/caregiver filling the Sinhala ATEC one week later, in a sample of 14 children. Intra-rater reliability was taken by com-paring 18 pairs of parents/caregivers Sinhala ATEC scores for the same child.Results: Sinhala ATEC showed high validity and reliability. Its concurrent validity has a strong positive Pearson correlation of 0.856 with the Consultant Psychiatrist assessment. Correlation was higher in total score and as well as in subscale II- Sociability score of the Sinhala ATEC. Internal consistency was revealed by Cronbach’s alpha 0.778. Cut off points to distinguish mild autism spectrum disorder from the rest taken as “48 or less” with sensitivity 82%, specificity 94% and area under the curve 0.968. Cut off to separate severe autism spectrum disorder from the rest were taken as “76 or more” with sensitivity 84%, specificity 90%and area under the curve 0.971. Reliability tests, intra class correlation was 0.98 and 0.96 for test-retest and intra rater reliability showed high reliability of Sinhala ATEC. For developing burnout in this study population.Conclusions: Translated version of Sinhala ATEC is suitable for the use in the Sri Lankan population as it shows high validity and reliability. However, further studies are recommended in other areas of the country to assess its usefulness across diverse subcultures and clinical settings.