The current study aimed to identify the effectiveness of the training program based on developing cognitive flexibility to reduce the severity of challenging behaviors in children with autism spectrum disorder, and to verify the extent of its continued effectiveness until the end of the follow-up period. The study tools included the Stanford- Binet Intelligence Scale, fifth edition (translated by: Mahmoud Abo El-nil),the Gilliam Assessment Scale for Diagnosing Autism, third edition(3-CARS)(translated by Adel Abdullah, Abeer Abu El-Majd), The Cognitive Flexibility Scale(prepared by: the researcher), The Challenge Behaviors Scale (prepared by: the researcher), the Socio-Economic and Cultural Level Scale of the family(prepared by: Mohammed Risk El Bahery), and a training program Based on developing cognitive flexibility to reduce the severity of challenging behaviors in children with autistic children (prepared by: the researcher). The pervious Scales were applied to a survey sample of autistic children consisting of (30) autistic children in Giza Governorate, to verify the psychometric properties of the study tools, and the scales were applied in their final form to a basic sample consisting of (20) children. The study used the experimental method, divided into two groups, one experimental group consisting of (10) and the other control group consisting of (10) autistic children whose ages ranged from (6-12) years, and using the following statistical methods: correlation coefficients, Mann-Whitney test, Wilcoxon test, and Eta square equation. The results of the study continued to the effectiveness of the training program based on developing cognitive flexibility to reduce the severity of challenging behaviors in autistic children, and the continuation of its effectiveness until the end of the follow-up period.