This study aimed to evaluate differences between low active overweight and obese children in terms of energy expenditure (EE), ventilation (VE), and cardiac response during graded submaximal treadmill testing at constant speed. Methods: We categorized 20 children into two weight groups according to the International Obesity Task Force criteria: overweight (n=10; age=9.7 ±1.34 years) and obese (n=10; age=10.4±1.4 years). Children performed treadmill testing at a constant speed (1.53 m·s-1 ) and increasing grade (0%, 4%, and 8%). every 3 min. Results: The EE across all grades was significantly higher (p<0.001) in obese than in overweight children. Differences at each grade disappeared when EE was adjusted by body mass; however, several differences remained when the EE was adjusted by fat-free mass or body surface area. The increase in EE with increasing grade was greater in obese children (effect size between 0% and 8% for EE was 1.17). BMI z-score and fat mass (kg) were the main predictors of EE (Kcal·min-1 ) and contributed to explaining 66%, 70% and 83.4% of the variance in EE at 0%, 4% and 8% gradients respectively. Conclusion: We suggest that when assessing EE response to exercise, the degree of obesity should be taken into consideration.