Incorporating nutrition topics in the primary school curricula should support the acquisition of nutrition knowledge in different ways and indirectly the development of healthy eating habits in children and teenagers. In Slovenia, nutrition education is part of all primary school education levels and may take the form of compulsory and/or elective school subjects. The main goal of the study was to find out the effectiveness of nutrition education of 6th graders where home economics is taught as a compulsory subject in nine-grade primary schools. The sample involved 630 pupils from 28 Slovenian primary schools. The nutrition knowledge was assessed by a knowledge test at the beginning and the end of the school year. The results show that the nutrition knowledge of students significantly improved. The students who scored lower on the test at the beginning of the school year made most improvement, and the students who scored best on the same test did not improve significantly. The results show that students' knowledge and understanding of the facts about energy and nutritive values of the foods is poor. Understanding a particular nutrient's role in human nutrition, or the energy value of the foods, requires certain abstract thinking ability, which the students, due to their cognitive developmental level, find difficult. Therefore, nutrition education should be adapted to the cognitive level of students and later, in the third triad of primary school, reinforced with interdisciplinary and constructivist approach. (Contains 4 tables.)