The topic of this work is Obesity of the younger school aged children. The goal of this work was to establish the prevalence of obesity of the younger school age children and to verify whether there is a significant difference between the sexes considering the status of body mass index and the body fat percentage. The research was conducted on the students from the first to the fourth grade in the primary school of Zadarski otoci in Zadar. In the research 88 student girls and boys were included. For the purpose of the research children from the first to the third grade were measured in body height, body mass and waist circumference. The children of the fourth grade were instead of the waist circumference, measured in the body fat percentage using the bioelectrical impedance. The results of the research indicate that the most of the tested students, more precisely, 68% have a normal body mass, the 22% have excessive body mass, and the 10% of the students belong in the overweight group. The research has shown that there are no significant differences in the body mass index considering the sexes. Almost similar body mass index increase was measured in the first and the second grade by both sexes. Between the second and the third grade there is a noticeable stagnation in the body mass index, while between the third and the fourth grade the body mass index increase has been measured with the girls. As the significance of the differences in the body mass index has been measured with the fourth grade students, the given results have shown that still there are no significant differences in the body fat percentage.