The aim of this study was to determine and validate the new thresholds for various intensities of physical activity in adolescents using the Actigraph accelerometer. Sixty healthy participants aged 10-16 years were recruited. Forty participants participated in the calibration study whereas the others participated in the validation study. Accelerometer data, heart rate, and oxygen consumption were measured at nine levels of physical activity which varied in intensity: sedentary, light, moderate and vigorous. The activity categories and accelerometer counts were sedentary activity, 0-400 counts/min; light activity, 401-1900 counts/min; moderate activity, 1901-3918 counts/min; and vigorous activity, greater than 3918 counts/min, respectively. This study establishes new threshold values for various physical activities that can be used for population-based studies in adolescents.