[Purpose] To determine inter-rater and intra-rater reliability using video recordings, and the concurrent validity of the Gross Motor Function Classification System (GMFCS). [Subjects] The subjects who were assessed were 71 children with cerebral palsy (CP; 46 boys, 25 girls) who were divided into three age groups according to the GMFCS guidelines; 0-2 years, 2-4 years, and 4-6 years. [Methods] Eleven pediatric physical therapists rated 71 video recordings of children with CP to test inter-rater reliability. Two of them rescored the same video recordings to test intra-rater reliability at an interval of one month. Concurrent validity was evaluated by comparing GMFCS levels with the subscale of the Pediatric Evaluation of Disability Inventory (PEDI) such as self-care, mobility, and social ability. [Results] By age group, inter-rater reliability was high (ICC = 0.994, 0.993, and 0.996 respectively). Intra-rater reliability was also high (ICC = 0.972-0.996), and correlation was higher between GMFCS level and the mobility domain of PEDI than between GMFCS level and non-motor domains of PEDI. [Conclusion] This study confirmed the reliability and validity of the GMFCS, supporting its use in clinical practice and research.