The present study evaluates the effect of a classroom-based behavioral intervention package to improve Japanese elementary school children's sitting posture in regular classrooms (N=68). This study uses a multiple-baseline design across two classrooms with a modified repeated reversal within each class. The article defines appropriate sitting posture as behavior composed of four components (feet, buttocks, back, and a whole body). The intervention package includes modeling, correspondence training, prompt, and reinforcement, among others. The authors counted the number of children with appropriate sitting posture in each classroom across all 28 sessions throughout the study. Interobserver agreement of appropriate sitting posture ranged from 80% to 100%. As a result of the intervention, the mean proportion of children with appropriate posture increased from approximately 20% to 90%. In addition, their academic writing productivity increased with the improved sitting posture. Teachers' acceptance of the intervention program proved to be excellent.