Abstract: Objective:Accelerometer can provide objective measurement of physical activity.The purpose of this study was to evaluate the validity of three contemporary activity monitors (GT3, RT3and SWA) under both laboratory and field conditions.Method:Thirty participants completed two kinds of routines designed to simulate a variety of lifestyle physical activities.Four different speed walking and running were completed under field conditions.Five different lifestyle activities were also examined to evaluate the validity of the monitors.During each routine, the activity levels of participants were monitored with the three activity monitors as well as by indirect calorimetry systems.Results:1) The validity of accelerometer is different for different types of physical activity.The validity of the walking and running were high compared with lifestyle activities;the measured error of upper limb activity is greater than lower limb activities;the energy changes from sitting to standing can not be recognized by accelerometers.2) Accelerometers tended to overestimate or underestimate the EE for the light intensity of PA, whereas overestimate the EE for vigorous activity.3) The validity among the different monitors were different for the same activity, Actigraph GT3was found to have better validity for walking (measured error less than 10%) , while SWA have better validity for lifestyle activities and running.Conclusion:The differences among the monitors were attributed primarily to differences activity type, intensity and monitors themselves.If you want to continuously measure the energy expenditure for a variety of activities (including walking, running and daily activities) , Sensewear Armband is preferred compared with GT3and RT3.