That patients have good motivation and cooperation during training process of physical therapy is very important for getting efficient effect of treatment. Unfortunately, it is difficult to well control the motivation and cooperation of patients during training process. For this reason, behavior modification has been used frequently in the training process of pediatric physical therapy in states for years. But it is not popular in Taiwan. To introduce the concept, principles and methods of behavior modification in pediatric physical therapy, the authors review the related articles and present a case report on one child with severely mental retardation. He was a nine-year-and-seven-month-old boy, coming for ambulation training. He had been brought to department of physical therapy for ambulation training twice before, but that didn't work due to his poor motivation and cooperation during training process. He could shuffle on bottom and walk with the support by two hands, But most of time, he just sat still and asked someone else to bring things what he wanted to him. He was not willing to move at all and unable to crawl and stand up with furniture and cruise. After treatment with behavior modification (including positive reinforcement and behavior shaping) and functional training, this patient was willing to walk and could walk independently with walker in one month. For some children with purely mental retardation, to facilitate the motivation to learn is the key point in the training of motor function. And that is the reason, behavior modification could motivale the child and make independently walking possible. Besides, to combine behavior modification in the training process of physical therapy for other pediatric patients-such as cerebral palsy, spinal bifida, mutiple handicapped children and children with chronic pain-to get greater effect was mentioned by other authors. Our experience indicates that well-applied behavior modification combined with physical therapy may be better in pediatric clinic than the traditional physical therapy, but it needs further study.