Objectives: This study was performed to assess the association between physical activity and the clustering of metabolic abnormalities among Korean children. The effect of substituting moderate to vigorous physical activity for the time spent in inactivity was examined as well. Methods: The study subjects were comprised of 692 (354 boys, 338 girls) 4th grade elementary school students. We used a modified form of the physical activity questionnaire that was developed in the Five-City Project. The subjects with clustering of metabolic abnormalities were defined as having two or more of the following five characteristics: waist circumference ${geq}90;%$, systolic or diastolic blood pressure ${geq}90;%$, fasting glucose ${geq}110;mg/dl$, triglycerides ${geq}110;mg/dl$ and HDL cholesterol ${geq}40;mg/dl$. We calculated the odds ratios to assess the effect of substituting moderate to vigorous physical activity for time spent in inactivity. Results: The risk of clustered metabolic abnormalities was inversely correlated with the increased time spent on moderate to vigorous physical activity, but the correlation was not significant. The odds ratio for clustering of metabolic abnormalities that represented the effect of substituting moderate to vigorous physical activity for 30minutes of sedentary activity was 0.87(95% Cl=0.76-1.01). Conclusions: These findings suggest that substituting moderate to vigorous physical activity for sedentary activity could decrease the risk of clustered metabolic abnormalities.