Objectivess To investigate relationship between iron deficiency of different degrees and physical performance and habitual activity of migrant schoolchildren at the age of 11-14 years. Methods Ninety one randomly selected schoolchildren were divided into three groups according to their iron status. Iron status including hemoglobin (Hb), serum ferritin (SF), serum iron (SI) and sTfR was determined. Physical performance tests included maximum oxygen consumption (VO2max) and maximum work time. Energy expenditure (EE) and daily physical activity were estimated by recording 24-h heart rate (HR). Dietary intake was assessed with frequency questionnaires, and physical activity level was estimated with frequency and physical activity questionnaires. Results Severe iron deficiency (IDA) impaired the aerobic capacity and habitual physical activity. When fat-free mass (FFM) was considered, VO2max (VO2max/FFM) was significantly lower in the iron-marginal group than in the iron-adequate groups among girls (P=0.02), but such a deference was not found among boys (p=0.28). Aerobic activity and EE at leisure were significantly lower in the severe iron deficient group than in the marginal iron deficient and iron adequate groups. Net HR at leisure time was correlated with Hb, log SF, body weight, and FFM (P