In this paper, we develop a module that combines a heart rate and movement sensor with embedded algorithm software for in-situ monitoring of human body energy cost. In previous studies, we have simulated the monitoring of energy expenditure in a lab environment; however, lab conditions are different from the free-living environment. The sensor algorithm was proposed to realize the real-time monitoring of energy cost in a free-living environment. Thirty college students (twenty males, ten females) were recruited to participate in this study. The participants walked and ran at a school playground without any speed limitations. The estimated values were compared with those of the portable metabolic system. The experiment results show the combined sensor module is able to estimate calories cost under free-living conditions.