The purpose of this study was to investigate the possible effect of the difference in percentage body fat (%BF) and fat-free mass (FFM) on cycling peak power (CPP) in 6- to 8-year-old obese and lean untrained girls and boys. Obese (35 girls, 35 boys) and lean (35 girls, 35 boys) children were measured for obesity, %BF, calculated from skinfold measurements. FFM was calculated as body mass (BM) minus body fat. A force-velocity test on a cycle ergometer was used to measure CPP. CPP was related to anthropometric variables using standard and allometric models. CPP in absolute terms was higher in obese children than in lean children irrespective of gender. BM-related CPP was significantly lower in obese children than in lean ones, whereas no effect of obesity appeared on FFM-related CPP. Velocity at CPP (Vopt) was significantly lower and force at CPP (Fopt) was significantly higher in girls than in boys. Muscle power production was unaffected by obesity in children. Low BM-related CPP could explain the difficulty of taking up physical activities that are body-mass related in obese children. Gender difference for Vopt and Fopt shows that girls and boys may have different maturation patterns affecting CPP.