An important goal of physical education (PE) is to educate children to become physically literate and active of their own accord. This study capitalized on examining the roles of students' attitudes toward PE and physical activity and fitness (PAF) knowledge in physical activity and sedentary behavior participation. Students from a public middle school in a Midwestern state in the United States of America responded to two surveys. One survey consisted of demographic and PAF knowledge questions for middle school students, and the other survey included the "Attitude Toward Physical Education Questionnaire" and the "Youth Activity Profile." Structural equation models showed that perceived enjoyment and perceived usefulness, as affective and cognitive dimensions of attitude toward PE respectively, demonstrated small effects on physical activity and sedentary behavior, while perceived enjoyment further showed a moderate effect on PAF knowledge, after controlling for gender and grade. These findings indicate the need to deliberately foster students' positive attitudes toward PE in middle school years. It is argued that future pedagogy research on youth physical activity should also include sedentary behavior as a focal variable.