In order to better understand individual's physical activity patterns and to design effective interventions, physical activity researchers have tried to search for accurate and reliable field devices to measure energy expenditure. An ankle-mounted activity monitor (AMP331; Dynastream Innovations Inc., Cochrane, Canada) was developed to measure steps taken, distance traveled, walking speed, and energy expenditure in and outside the laboratory environment. The accuracy of energy expenditure estimation using AMP331 device under field environment, however, has not been well established. The purpose of this study was to validate the accuracy of the AMP331 monitor in quantifying energy expenditure of free-living physical activity. Diary method, a criterion measure, was used to validate the AMP331 monitor, and energy expenditures in kcal were obtained from both diary and AMP331 data. Forty-one 8th grade middle school children (20 boys and 21 girls) were recruited from Korea for this study. Participants were instructed to record their daily physical activities into a recorder whenever they engaged in a new activity from 8:30 am to 8:30 pm while wearing the AMP331 monitor in the right ankle. Energy expenditure from recorded data was calculated by the primary investigator. Pearson correlation coefficient and paired t-test were performed between two measures, and the significant level was set at .05. The results showed that children's daily energy expenditures were 1970.13 kcal (SD = 275.9) from diary data and 1956.05 kcal (SD = 378.5) from AMP331 data. The AMP331 monitor tended to slightly underestimate energy expenditure against energy expenditure obtained from recorded data. The Pearson correlation coefficient between diary and AMP331 measures were moderately high (r = .651), and no statistically significant difference was found between two measures (t = .312, p = .757). The statistical results indicated that the AMP331 monitor is a valid tool for assessing energy expenditure of free-living physical activity. It is expected that the AMP331 monitor become a practical field tool for the intervention research to effectively measure individual's energy expenditure under field environment.Keyword(s): measurement/evaluation, physical activity, research