Since 1985, the American Heart Association has maintained a schoolsite program that has made preventative health education available to over 76 million students. This study proposes to measure the implementation and effectiveness of the American Heart Associations Heart Power Program, which is a descendant of The Heart RX Program developed in 1985. This study was investigated the effects of the American Heart Association Schoolsite's Heart Power curriculum on fourth and fifth grade students' knowledge, attitude, and physical activity behavior in physical education classes. Six fourth and fifth grade elementary school classes (n=245) and their physical education teachers from two separate schools participated in this study. One was involved as a treatment group and the other was used as a comparable group. The researcher provided the Heart Power curriculum to the physical education teacher in the treatment group to teach health-related fitness information to elementary school students for four weeks while the comparable group was participating in the usual physical education curriculum. For this study, only two health topics were selected (Healthy Heart and Physical Activity topics) because the teacher's class schedule was limited. Heart Health Cognitive Test (HHCT), Children's Attraction to Physical Activity questionnaire (CAPA), and the System of Observing Fitness Instruction Time (SOFIT) were used to collect the data for this study. Each class completed the HHCT test and CAPA questionnaire three times during the study: (1) before the intervention, (2) at the end of the intervention (four weeks), and (3) once on a follow-up four weeks after the study. Student's activity levels and teacher's behavior during the class were measured with SOFIT. Teacher Satisfaction survey and informal interviews were used to collect the teacher's opinions about the implementation the Heart Power lessons into her classes. The results indicated the students in the treatment group had significantly higher scores on the knowledge test in the post- and follow-up tests than the comparable group. The results also indicated a significant difference in the level of attraction toward physical activity in liking of exercise, liking of games and sport, perceived health benefits in the post- and follow-up tests between two groups. No difference was found between genders in both groups in the knowledge test and the attraction toward physical activity. The results indicated that the Heart Power curriculum is associated with improvements in health-related fitness knowledge and the children's attitude toward physical activity in fourth and fifth grade students.