Objective: This study tested the validity and reliability of the Behavior Development Screening for Toddlers-Questionnaire-Parents (BeDevel-Q/P), a new autism spectrum disorder (ASD) screening instrument being developed in South Korea. The parents of 24-35-month-old infants were recruited to complete the questionnaire. Methods: The participants were 791 infants aged 24-35 months. There were 623 typically developing infants, 88 infants with ASD, and 80 developmentally delayed infants. For test-retest, the participants were surveyed every 1-4 weeks. Participants were recruited nationwide. Subjects’ parents completed the BeDevel-Q/P and concurrent validity questionnaires. The data were used for statistical analysis. Results: A total of 24 items consisting of 16 items from factor 1 (F1), 6 items from factor 2 (F2), and 2 items from factor 3 (F3), were selected for the final BeDevel-Q/P items. Conclusion: The factors of the screening instrument developed in this study were analyzed, and three factors were extracted, confirming the theoretical foundation of the BeDevel-Q for the parents of 24-35-month-old infants.