The purpose of this study was to explore the impacts of physical fitness lessons on elementary school students' academic performances, in which the behavior of sports participation is treated as an impact factor. Participants were the fifth and sixth grade students in the 100th academic year the Elementary School in Taichung. Surveys were conducted through comprehensive sampling, and a total of 543 questionnaires were collected as valid data. Descriptive statistics are done by SPSS, cross analysis, and Pearson correlation methods to identify the relationship between the variables. Hierarchical multiple regression analysis was used to explain the predictions of the relationships between components of fitness and academic achievements. The behavior of sports participation acted as an interfering variable of the prediction. The result of the study was concluded as follows: Firstly, there is a partly predictable relationship between health fitness and academic achievements of Grade five and grade six students in Elementary School. BMI values of physical fitness showed a negative connection with students' academic performances, which meant the higher BMI value (worse body composition), the worse the academic performance was. The ability of doing seated forward bend was positively connected with students' academic performances, which meant those who could reach farther while they sat and bent forward scored better academically. Secondly, sports participation behaviors of grade five and grade six students in Elementary School showed a moderate effect on their academic performances. Students who participated longer and more frequently in sports, achieved better scores academically. Students who performed better in sports also tended to have better academic performances.