Obesity is one of the leading diseases today, and it is the consequence of collecting too much fat in the body. According to the World Health Organization (WHO), obesity is spreaded worldwide and approximately 1,5 million people are overweight. It is being predicted that obesity will grow, which is a trend Croatia is also not spared of. Sedentary way of life, unhealthy eating habits and availability of fast and unhealthy food are considered to be the main reasons for obesity (Medanić, Pucarin-Cvetković, 2012). The main objective of this research was to establish the body mass index (BMI) of the students and to determine if there are any significant differences between the BMI of male and female students, who attend a primary school in Krapinsko-zagorska županija. The participants were students from 1st to 4th grade. Also, another objective was to check if there are any connections between the eating habits and physical activity with overweight and obesity according to the sex of the participants. For the purpose of the study, the weight and height of the students were measured to calculate the BMI. The students also filled a questionnare, which consisted of questions related to eating habits and physical activity. The results showed that there is no big difference between the participants' BMI. The percentage of overweight children is similar in both groups, while there is around 18% more boys who are overweight. According to the answers of the questionnare, boys don't have the habit of eating breakfast before school or during the weekend, they drink more juice than regular water, they often eat fast food, they spend more time daily using gadgets, and they spend most of their free time using the gadgets. Girls also often tend to eat fast food, they eat sweets more often, and they spend less time doing sport. The given results show that there is no significant difference between eating habits and the level of physical activity with overweight and obesity among these students.