[Purpose] This study aimed to investigate the effectiveness of neurodevelopmental treatment-based physical therapy on muscle tone, strength, and gross motor function in children with spastic cerebral palsy. [Subjects and Methods] One-hundred-seventy-five children with spastic cerebral palsy (88 diplegia; 78 quadriplegia) received neurodevelopmental treatment-based physical therapy for 35 minutes per day, 2-3 times per week for 1 year. Spasticity, muscle strength, and gross motor function were measured before and after treatment with the Modified Ashworth Scale, Manual Muscle Testing, and Gross Motor Function Measure, respectively. [Results] Spasticity was significantly reduced after 1 year of treatment. The Gross Motor Functional Classification System levels I-II group showed a significant increase in muscle strength compared with the Gross Motor Functional Classification System levels III-V, and the latter showed a significant decrease in spasticity compared with the former. [Conclusion] Neurodevelopmental treatment-based physical therapy in children with cerebral palsy seems to be effective in reducing spasticity, but does not improve gross motor function. Therefore, other interventional approaches are needed to improve gross motor function in children with cerebral palsy.