Children with developmental disabilities tend to engage in less physical activity than neurotypical children. In this study, a classroom staff implemented physical activities (structured physical activity (SPA), free play physical activity (FPA) and sedentary activity (SA) intervention in a self-contained classroom to increase on-task behavior among children with autism spectrum disorder (ASD) in the context of an alternative treatment design. On-task behavior and problem behavior were observed for three children with autism spectrum disorders. The students engaged in a 10-minute session of physical activity followed by 10-minute of Discrete Trial Training (DTT). Furthermore, the researcher collected problem behavior as a secondary variable to determine whether there was a relationship between physical activity and problem behavior. Results showed a functional relationship between higher levels of on-task behavior and lower levels of problem behavior in one of the students. These findings suggested that for some students engaging in physical activity before academic instruction might increase on-task behavior.