Children’s motivation towards Physical Education (PE) decreases over the elementary school year, but little is known about how and why those declines influence changes of children’s intention and Physical Activity (PA). Guided by the expectancy-value model, this study attempted to examine the relationship between changes of children’s motivation to changes in children’s intention for future participation in PE and PA over an academic year. Using a repeated measurement technique,school. children’s expectancy-related beliefs, task values, intention, and PA were assessed twice over one academic school year. Time 1 was at the beginning of the fall semester, and Time 2 was at the end of the subsequent spring semester. Students’ expectancy-related beliefs and task values were positively related to their intention and PA at both Time 1 and Time 2. A factorial repeated measure MANOVA showed that decreases in task values and intention were significant. Regression analyses indicated that decreases in children’s expectancy-related beliefs and task values explained significant variance in a decrease in their intention. Additionally, changes of expectancy-related beliefs emerged as significant predictors of changes in children’s PA. The findings revealed that children perceived less values toward PE across one academic year. Consequently, this change could contribute to the decreases in children’s intention for future participation. Thus, PE teachers should focus on learning activities students perceive as important, interesting, and usefulness in elementary school.