Expectancy beliefs and task values are two essential motivators in physical education. This study was designed to identify the relation between the expectancy-value constructs (Eccles & Wigfield, 1995) and high school students' physical activity behavior as associated with their energy balance knowledge. High school students (N = 195) in two healthful-living programs (i.e., combination of physical and health education) responded to measures of expectancy-value motivation, energy balance knowledge, in-class physical activity, and after-school physical activity. The structural equation modeling confirmed positive impact from expectancy beliefs and interest value to in-class physical activity (Path coefficient range from 0.19 to 0.26, ps less than 0.01). Cost perception was found exerting a negative impact on after-school physical activity but a positive one on lower level of understanding of energy balance (Path coefficient range from -0.33 to -0.39, ps less than 0.01). The findings painted a complex but meaningful picture about the motivational impact of expectancy-value constructs on physical activity and energy balance knowledge. School healthful-living programs should create motivational environments that strengthen students' expectancy beliefs and interest value and alleviate their negative perceptions and experiences. (Contains 4 tables and 1 figure.)