Background: Autism spectrum disorder (ASD) is a neurodevelopmental disorder where symptoms first appear during the first two years of life. Early screening and diagnosis are the keys to identifying toddlers with ASD so that early intervention could be recommended. The Modified Checklist for Autism in Toddlers, Revised with Follow-up (M-CHAT-R/F) is a worldwide ASD screening instrument with adequate validity, which is used to screen ASD in toddlers between the ages of 16 and 30 months. Objective: To compute the sensitivity and specificity of the Thai version of the M-CHAT-R/F on the Autism Screening application (AS app) among children ranging from 16 to 30 months of age. Materials and Methods: The permission to translate M-CHAT-R/F into Thai and place it on an AS app on the computer website, on iOS, and on Android smartphones/tablets was granted. The forward translation of M-CHAT-R/F to Thai and the back translation into English were conducted by the language experts. The content was proofed and edited by two Child and Adolescent Psychiatrists and by one Developmental Pediatrician. The final Thai version of the M-CHAT-R/F was placed in the AS app and was used in the pilot study, which consisted of 15 toddlers with ASD and 15 toddlers with typical development. The screening results from the AS app were compared to the diagnoses from either Child and Adolescent Psychiatrists or Developmental Pediatricians to calculate the sensitivity and specificity. Results: The number of female(s) and male(s) among typical toddlers and toddlers with ASD were 1/14 and 2/13. The mean ages were 16 (SD 0)/26 (SD 4.31) and 23 (SD 9.90)/29.08 (SD 1.71) months old. The Thai M-CHAT-R/F gave the sensitivity and specificity of 100% at the cutoff score of 6 or greater from the full score of 20. Conclusion: The AS app had a high level of validity at the cutoff score of 6 or greater. Keywords: Application; Screening; Autism; Toddlers; Thai