The object of this study is to examine the effect of cognitive behavior program on elementary school students’ aggressiveness when the program is applied to Taekwondo practices. The target group of the study is 12 boys, who started learning Taekwondo and are enrolled in first year of elementary school, located in Daegu province of Korea. During their Taekwondo practices, twice a week for 10 weeks, thus, in total 20 sessions of cognitive behavior program was conducted. The aggressiveness and K-CBCL test was carried out both before and after the program. The SPSS 20 program was used to process the collected data. To verify the construct validity of the collected data, exploratory factor analysis was used and to check the reliability, reliability test was conducted. The results indicate that overall aggressiveness, physical aggressiveness, verbal aggressiveness, indirect aggressiveness, pessimistic personality and excitement decreased after 20 sessions of the program. Overall aggressiveness (p=0.003) and physical aggressiveness (p=0.026) verbal aggressiveness (p=0.007) show statistically significant difference between before and after the program. K-CBCL decreased after the program, and was statistically significant (p=0.027). The following conclusion may be drawn from the results. Frist, cognitive behavior program through Taekwondo practices have significant effect on decreasing elementary school students’ aggressiveness, especially physical and verbal aggressiveness. Second, the program also has decreasing effect on K-CBCL. That is, cognitive behavior program through Taekwondo practices help release stress by way of physical exercise, help students to control their emotions and to find solutions to their problems. Therefore, programs which may enhance communication between parents and children, and are easily understandable by students need to be developed.