Background: Recently, the prevalence of childhood obesity has increased dramatically, but the cause still remains incompletely understood. The goal of this study was to present a direction for treatment and prevention of childhood obesity on the basis of the data of physical activity energy expenditure, inactive time and dietary habits of Korean children. Methods: A total of 56 children were selected from 8 elementary schools in Seoul and Gwacheon-city. Children's physical activity pattern and dietary habits were obtained by questionnaire. The children wore accelerometer on their wrists from Friday to Tuesday over the whole weekend. Then, we calculated the activity energy expenditure using the activity counts recorded on the accelerometer. Results: Neither the physical activity energy expenditure (kcal/kg) nor the activity pattern showed a significant difference between the normal weight group and the overweight group. The overweight children spent more time watching TV and using the computer. The overweight group did not have breakfast and ate much more and faster in terms of dietary habit. Conclusion: This study suggests that bad habituations in diet and increase of inactivity time can play more important role than physical activity in causing overweight in 7-year-old children.